This isn't a drop in support so much as a reversion.

Biden was only favored among 20% of young voters during the primaries, trailing Sanders by 11%. Sanders, O'Rourke, Warren, and Yang, all perceived as progressive candidates, combined for 47% of young primary voters. The rest of the candidates, who were either seen as moderate or just plain weird, combined at 33%. 20% were undecided. It's reasonable to assume at least some of the folks who preferred other moderate candidates also had a low opinion of Biden.

Now, 29% of young people approve of Biden, while 55% disapprove, and 17% don't know. Take your primary voters who wanted a progressive president, add in a few undecideds and a handful of moderates who don't like Biden, and there's your approval rating.

Last January's approval numbers were an anomaly, most likely sparked by that whole attempted insurrection thing, and also that brief window of time where it looked like congressional Democrats had actually found their spine.