When did young voters begin to favor the Democrats?

[Virginiá]

I believe during the Clinton era, but their support wasn't truly locked in until 2004. Clinton left a positive mark on young voters during his second term, but demographic and other changes really began to tilt that bloc towards Democrats in the mid-2000s. Now the weight of racial minorities and a substantially less Republican-leaning generation of white voters have created an age group largely hostile to Republicans.

[Virginiá]

According to Gallup, young voters have always been more Democratic.

In 1936, FDR won 68% of voters under 30 and 56% of voters 50+.

In 1948, Truman + Thurmond + Wallace won 62% under 30 and 50% over 50.

In 1952, Stevenson won 51% under 30 and 39% over 50.

In 1972, McGovern won 48% under 30 and 36% over 50.

In 1980, Reagan won 41% under 30 and 54% over 50.

In all of the above years, the gap was at least 12 points. In other years, the gap was smaller, but younger voters generally voted more Dem than older ones.

I think 2008 set the record for difference between 18-29 and 50+, but 2016 may be a close second, especially if we lump Clinton+Johnson+Stein analogous to what Gallup did for 1948.

As a side note: in 1972, when George C. Wallace was a candidate, his support was approximately the same for those 21-29, 30-49, and 50+ -- but dropped way down for 18-20 year olds.

Reagan won them in 1984, carrying around 60%, and Bush I carried them by around 6 points. Further, Carter carried young voters by a very tiny margin - something like a couple points or so. To be honest, I'm a bit skeptical about relying on those age polls from the 30s - 50s.

I really don't think the idea that today's youth are behaving just the same as before would be correct. Not at all.

[Virginiá]

At least according to exit polling (which are not gospel on this sort of thing but generally backed up by poll subsamples at the time so reasonably believable), Bush narrowly won 18-25-year-olds in 2000. Thus, they were marginally more Republican than the nation as a whole, as Gore overall won the popular vote very narrowly. There is generally no evidence that younger voters were voting more for the Democrats than older voters in the 90s or earlier as well.

If you want to go by how much more Democratic they voted compared to the nation, then sure I suppose in 1992/1996 you could say it was rather unremarkable, but I don't really see it that way. How voters vote in their youth can be highly suggestive of their future, and in Clinton's case, those voters have displayed modest Democratic leanings even as they aged. Bill Clinton was a popular president who presided over a booming economy, so the idea that young people growing up under him may have taken a shine to Democrats is not far-fetched. All presidents, good or bad, have an effect on the young people who grow up under their tenure.

What you said about Bush winning some portions of younger voters, sure, I think that could suggest that Clinton's impact among 90s youth was more isolated to him and the circumstances of the time, and was not really connected to the main trend that caused young people to begin heavily trending Democratic in 2004-on.