I mean...yeah. Young people who are on the right don't like Biden because they are on the right.
Young people on the left don't like Biden because he is too far right for them. A lot of young people support Bernie/AOC and believe Biden has failed to deliver a progressive agenda.
So he only really ever had support in the first place from the tiny minority of young moderates.
In general, if you look at the world's liberal democracies, centrism is under attack. Look at France, for example. In a culture war, one side brings their generals and foot soldiers, and then the side that feels under attack wants to do the same - they don't want compromisers and deal makers, which is what Biden is.
The pandemic and its aftermath have been a time of extreme trauma and disruption, so the chances that a dealmaker and compromiser could satisfy anyone are slim. People feel attacked, and they want to counter-attack. They're in no mood to sit down to have dinner with the 'devil' sitting across the table from them.
To be honest, I think - always did feel - Biden was too old to be president. And there is such a thing. Trump was also too old. Hillary was also too old. People need to see energy when they see the nation's highest political leader. They need to see energy, optimism, pragmatism, adaptability. They need to have an understanding of history and how we got here but it's far more important that they recognize what contemporary thoughts and feelings are, particularly from the perspective of the people who are actually working and paying the taxes for the social security and medicare recipients and the generation that is just now beginning to step into middle and upper management roles.
I think if you're still young enough to have children just graduating from university or starting out in the workforce, you're probably still connected enough with that younger generation to have some perspective. If you're in your 50s or even your 60s, you're still working along side or managing people in their 20s and 30s - you can still 'get them' on some level even if you privately think they're snowflakes, don't like their taste of music, and get bent out of shape because they 'ghosted' you. But if you're a generation removed from that, meh, I think that's too old.
Biden, Bernie, Trump, Hillary...too old.