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

[TML]

This is mostly because of the political leanings of different generations. The Greatest Generation (people born 1901-1927) was a D-leaning cohort, while the Silent (1928-1945) & Boomer (1946-1964) generations were R-leaning cohorts, which would explain the rightward shift among the 65+ voting bloc. On the other hand, older Generation X (1965-1980) people are R-leaning, while younger ones along with most millennials (1981-1996) are D-leaning, which explains the leftward shift among the under 25 voting bloc.

[TML]

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.

Also, young people have trended pretty substantially back to the right since the Obama years, when the "hope and change" messaging was more powerful.

Keep telling yourself that. You've admitted you live in a bubble before, and this it it coming through again. Abortion rights and such are as popular as ever among younger people. In 2012, Obama won those 18-24 by a 60-36 margin (D+24) and Biden won them by 34 points. Some rightward shift.

2008: D+34/D+33 (18-24/25-29)
2012: D+24/D+22
2016: D+21/D+14
2020: D+34/D+11

As for 2020, I have a hard time believing that big of a dichotomy between 18-24's and 25-29's.  I could buy that some gap has opened up, but I have a hard time seeing an over 20-point difference between those voter groups.  I'm guessing it's just small wonky sample sizes.  Like, in 2020, the "white 18-29" vote was either R+9 or D+7, depending on which exit poll you use.

I was wrong about 2012, but it still seems that overall 2008 levels of support for Democrats among young voters was an anomaly.  Additionally, it's a myth that people don't get more conservative as they age.  The 18-29 crowd from 2008 would have been 30-41 in 2020.  According to those same exit polls, people in their 30s were only D+5, or one point to the left of the NPV, in 2020.  Going from over D+30 to an even PVI seems notable.

It's not that people get more conservative as they age, it's that large groups of young people 18-29 simply don't start participating in elections until their 30's, and it's mostly young people without college degrees that fall into that category.  

The trends are entirely explainable by the young people who sit out of elections in their 18-29 years being overwhelmingly Republican-leaning.

Edit - Also you have to adjust for 2008 being a 7.2% win and 2020 being a 4.5% win.

There are also some Obama voters who decided to sit out subsequent elections because they felt he did not meet their expectations after he entered office. That would also decrease the share of people voting for his party and thereby increase the share of people voting for the opposing party.

[TML]

Young voters were fairly conservative in previous years.  Didn't they vote heavily for Nixon in 1972 and Reagan in 1984? 

True, but in both cases youth support for Republicans was weaker than senior support for Republicans (per exit polls):

1972	D	R
18-29	46	52
60+	31	68

1984	D	R
18-29	40	59
60+	39	60

In fact, the only election cycles where exit polls showed stronger Republican support among the youngest voting bloc (under 30) compared to the oldest voting bloc (over 60) were 1988, 1992, and 2000 - this reflected just how D-leaning the Greatest Generation was compared to the subsequent three generations that followed.

It may be some social issues today and social trends that have caused younger generations to have a very different attitude towards certain things than their grandparents did.  I know with my grandparents, I would differ from them enormously on issues such as interracial marriage and such.

According to Pew Research, younger generations are more diverse compared to older generations, and these people thus have more opportunity to interact with people whose demographic backgrounds differ from their own.