If you mean uninhabited by any form of life, that's only determined for the Moon and in that case it was thought it could have life right up to Apollo 11 definitely.
Carl Sagan was concerned that the Apollo 11 astronauts got out of their capsule with the hatch open to the ocean meaning that any life from the Moon that had got into say the dust in the capsule could have escaped into the ocean and who knows what it would have done to the ocean.
I think it is reasonably obvious that if there was any microscopic dormant life in the lunar dust that had the capability to live in the Earth oceans, it would have got into it at that point, because they had a lot of dust on their clothes, in the spacecraft etc.
So anyway, the point is that Carl Sagan's concern shows that at that point it was thought to be still a chance, I think most would say a remote chance - but one that was important to guard against even so, because of the potentially huge outcome if we did return something harmful to the environment of the Earth or to human health.
The law about lunar quarantine to protect the Earth was withdrawn in 1971. Extra-Terrestrial Exposure Law
So I suppose you could say for the Moon was generally accepted it was lifeless some time around 1971. So that's quite a narrow window. Some time between 1969 and 1971.
Mars is still potentially habitable for microbes. After Viking in the mid 1970s most scientists thought that it was probably not inhabitable on the surface, with the exception of Gilbert Levin who continues to this day to maintain that there's a possibility his Labelled Release experiment did detect life back then.
But since 2008, then with measurements by Phoenix, and later, various observations of seasonal effects, a very few of which still have no other explanation except water - then opinion has swung the other way - now have a full spectrum of views from some who continue to say it is uninhabitable - to some who think there is a very good chance of finding present day microbial, possibly even plant life like lichens, on Mars. And others take various intermediate views.
Venus surface was shown to be uninhabitable for Earth type life with the flyby of the Russian Mariner 2 in December 1962
Mariners to Venus
Before then I think you could have held out hope for a habitable world of some sort beneath its obscuring clouds.
Venus upper atmosphere may be potentially habitable however. It's in some ways the easiest place to build human habitats, floating because our breathable atmosphere is a lifting gas a bit like hydrogen there. But it may also, just possibly, have microbial life. Because it would take several months for a microbe to fall through the atmosphere.
But as for intelligent life, well none of these, except Earth, seem promising.
I think for intelligent life - for Venus again I'd go for december 1962.
For the Moon, in 1835 then the great Moon hoax caught out many - when the Sun, in UK, claimed that these were drawings of the Moon taken through a super high powered new telescope in the Southern Hemisphere - by John Herschel one of the most famous astronomers of his day. He was away in the Southern Hemisphere making observations in his observatory, so couldn't contradict the story (didn't have the easy global communications then that we do now, well before invention of radio and even Electrical telegraph was in its early stages, first 5 kilometer test cable in 1836).
Of course this was way way beyond any capability of any Earth based telescope then or indeed up to the present day also. But many were taken in and believed the hoax.
And - they had papers published with ideas of how to communicate with beings on the Moon and other planets for instance by digging ditches in the deserts in the shape of geometrical diagrams and filling them with fire at night.
Back in 1838 one astronomer had already concluded, correctly, that the Moon because it had such a sharp edge in his measurements, had no atmosphere and so therefore, he concluded no water, and no fire. From which life would seem unlikely.
Religions and Extraterrestrial Life
By the late nineteenth century I think generally agreed by scientists that there was no lunar atmosphere. For a while they got confused by the solar corona which you see only when the moon is in front of the sun, but gradually it became clear that it is an "atmosphere" of the sun, rather than the moon.
I think by the early C20 if you were a scientists, you'd have been thought very eccentric to expect intelligent life on the Moon, already known almost no atmosphere.
For Mars - the observations in the Leck laboratory in 1926 showed there is very little oxygen - that started it. But you could still think that there is life that makes far more efficient use of whatever little oxygen there is.
So, if there is intelligent life there, it must have a very different basis from Earth life.
I think the Mariner 4 flyby in 1964 pretty much put an end to most people's hope for finding some kind of super efficient intelligent life there.
After they got the images back like this:
it was pretty obvious it wasn't very habitable.
Partly it was because they looked at the least habitable part of Mars. Just by chance its only images were of the highly cratered uplands.
Anyway - that was maybe the lowest point of Mars habitability assessment indeed :). If anyone in the scientific community still had the thought that there might be some forms of intelligent life on the Mars surface somehow subsisting in the near vacuum atmosphere there - I think they would probably have given up on it at that point.
So - we have only one example of life, so not easy to prove anything conclusively. But I think for intelligent life, for most people, Mars in 1964 with Mariner 4, Venus in 1962 with the Mariner 2, and the Moon - 1870s??
For microbes: Mars - not yet - depends who you ask, and some would think it may well have plant life as well, especially lichens (though with a dip between 1970s and 2008 when many thought its surface at least was totally uninhabitable), Venus - 1962, but upper atmosphere still a possibility, and Moon - some time between 1969 and 1971 for most people.
BTW we could still have intelligent - non technological - life in the oceans of Europa especially because it is oxygen rich and may be that it has plenty of organics and hydrothermal vents - if so it may easily have advanced multicellular life and if it had intelligent life how would it develop technology without fire? So we wouldn't know about them yet, and they wouldn't know about us.
Carl Sagan was concerned that the Apollo 11 astronauts got out of their capsule with the hatch open to the ocean meaning that any life from the Moon that had got into say the dust in the capsule could have escaped into the ocean and who knows what it would have done to the ocean.
I think it is reasonably obvious that if there was any microscopic dormant life in the lunar dust that had the capability to live in the Earth oceans, it would have got into it at that point, because they had a lot of dust on their clothes, in the spacecraft etc.
So anyway, the point is that Carl Sagan's concern shows that at that point it was thought to be still a chance, I think most would say a remote chance - but one that was important to guard against even so, because of the potentially huge outcome if we did return something harmful to the environment of the Earth or to human health.
The law about lunar quarantine to protect the Earth was withdrawn in 1971. Extra-Terrestrial Exposure Law
So I suppose you could say for the Moon was generally accepted it was lifeless some time around 1971. So that's quite a narrow window. Some time between 1969 and 1971.
Mars is still potentially habitable for microbes. After Viking in the mid 1970s most scientists thought that it was probably not inhabitable on the surface, with the exception of Gilbert Levin who continues to this day to maintain that there's a possibility his Labelled Release experiment did detect life back then.
But since 2008, then with measurements by Phoenix, and later, various observations of seasonal effects, a very few of which still have no other explanation except water - then opinion has swung the other way - now have a full spectrum of views from some who continue to say it is uninhabitable - to some who think there is a very good chance of finding present day microbial, possibly even plant life like lichens, on Mars. And others take various intermediate views.
Venus surface was shown to be uninhabitable for Earth type life with the flyby of the Russian Mariner 2 in December 1962
Mariners to Venus
Before then I think you could have held out hope for a habitable world of some sort beneath its obscuring clouds.
Venus upper atmosphere may be potentially habitable however. It's in some ways the easiest place to build human habitats, floating because our breathable atmosphere is a lifting gas a bit like hydrogen there. But it may also, just possibly, have microbial life. Because it would take several months for a microbe to fall through the atmosphere.
But as for intelligent life, well none of these, except Earth, seem promising.
I think for intelligent life - for Venus again I'd go for december 1962.
For the Moon, in 1835 then the great Moon hoax caught out many - when the Sun, in UK, claimed that these were drawings of the Moon taken through a super high powered new telescope in the Southern Hemisphere - by John Herschel one of the most famous astronomers of his day. He was away in the Southern Hemisphere making observations in his observatory, so couldn't contradict the story (didn't have the easy global communications then that we do now, well before invention of radio and even Electrical telegraph was in its early stages, first 5 kilometer test cable in 1836).
And - they had papers published with ideas of how to communicate with beings on the Moon and other planets for instance by digging ditches in the deserts in the shape of geometrical diagrams and filling them with fire at night.
Back in 1838 one astronomer had already concluded, correctly, that the Moon because it had such a sharp edge in his measurements, had no atmosphere and so therefore, he concluded no water, and no fire. From which life would seem unlikely.
Religions and Extraterrestrial Life
By the late nineteenth century I think generally agreed by scientists that there was no lunar atmosphere. For a while they got confused by the solar corona which you see only when the moon is in front of the sun, but gradually it became clear that it is an "atmosphere" of the sun, rather than the moon.
I think by the early C20 if you were a scientists, you'd have been thought very eccentric to expect intelligent life on the Moon, already known almost no atmosphere.
For Mars - the observations in the Leck laboratory in 1926 showed there is very little oxygen - that started it. But you could still think that there is life that makes far more efficient use of whatever little oxygen there is.
So, if there is intelligent life there, it must have a very different basis from Earth life.
I think the Mariner 4 flyby in 1964 pretty much put an end to most people's hope for finding some kind of super efficient intelligent life there.
After they got the images back like this:
Partly it was because they looked at the least habitable part of Mars. Just by chance its only images were of the highly cratered uplands.
Anyway - that was maybe the lowest point of Mars habitability assessment indeed :). If anyone in the scientific community still had the thought that there might be some forms of intelligent life on the Mars surface somehow subsisting in the near vacuum atmosphere there - I think they would probably have given up on it at that point.
So - we have only one example of life, so not easy to prove anything conclusively. But I think for intelligent life, for most people, Mars in 1964 with Mariner 4, Venus in 1962 with the Mariner 2, and the Moon - 1870s??
For microbes: Mars - not yet - depends who you ask, and some would think it may well have plant life as well, especially lichens (though with a dip between 1970s and 2008 when many thought its surface at least was totally uninhabitable), Venus - 1962, but upper atmosphere still a possibility, and Moon - some time between 1969 and 1971 for most people.
BTW we could still have intelligent - non technological - life in the oceans of Europa especially because it is oxygen rich and may be that it has plenty of organics and hydrothermal vents - if so it may easily have advanced multicellular life and if it had intelligent life how would it develop technology without fire? So we wouldn't know about them yet, and they wouldn't know about us.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.3k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0