Many young voters are registered to vote under the Motor Voter law, which as implemented in most states, does not establish a party identification when they are automatically registered to vote upon obtaining a driver's license. So, they are never affirmatively required to take the act of identifying with a party as voters historically had to do.
It also reflects a generational rejection of existing institutions, more generally, including political ones.
But this is purely a branding issue. Despite not identifying as Democrats, young voters continue to vote for Democrats in proportions greater than this age group did in 2008.
For example, in the two main 2020 Presidential election exit polls:
The Votecast (CNN) breakdown of Biden support by age was:
18-29 61% (60%)
30-44 54% (52%)
45-64 48% (49%)
65+ 48% (47%)
CNN also had a different age breakdown with Biden support as follows:
Exit polling by CNN in 2022 reflects the same general trend:
(The bump between 18-24 year olds and 25-29 years olds in the 2022 exit polling, given the subsample sizes, is just barely statistically significant, and both age groups under age 30 still trend more Democratic in how they actually vote than any age group aged 30 and up.)
It isn't just in the Presidential election particularly pitting Biden against Trump either. Generally speaking, younger voters in the U.S. at this time are more liberal than older voters in almost every dimension you can measure with polling.