2000: 65+ voters were D+4, 18-24s were tied. 2016, 65+ were R+8, 18-24s were D+21. What changed?

[Bootes Void]

The Bush presidency and the subsequent Obama presidency  made a lot of younger people a lot more liberal. Basically before the 2004 election, there was very little correlation between youth and partisanship

And for the most part the bush administration made strong appeals to older voters with Medicare part D and not pursuing fiscal austerity as some republicans wanted to do so.