I highly recommend this
NYT article for a great look at both historical (post-ww2) voting and what I'll call the "popular president" demography theory.
My own view is that younger voters probably will become more conservative as they age, (hopefully) gain income, and "tsk tsk" at the values of younger people. But I think that, between having two relatively popular Democratic presidents (Clinton+Obama) and two relatively unpopular Republican presidents (Bush+Trump), a high percentage of non-whites and first/second/third gen immigrants, and difficult economic circumstances caused in large part by income/wealth inequality, there's a really good chance that millennials and Gen Zers will be the most economically left-leaning generation since at the very least the Greatest Generation.
With that said, it seems possible that, in the long term, greater liberalism won't necessarily equate to these generations becoming solidly Democratic. Such a scenario would (I hope) probably require the Republican Party to moderate at least a little, but it's also possible we see a depolarization of the parties (similar to the post-war era), or a Democratic Party that really moves left and alienates the middle.
Poll
