"Doing well" is always relative depending on who you ask. Beyond his age, the thing that's hurting Biden the most is a lingering perception that prices were cheaper when Trump was in office, which goes back to the old Reagan question "are you better off today than you were four years ago".
Because of inflation, the answer is usually no, and even though Biden has brought inflation down over the past year or so, the cost of buying food is still significantly higher than when Trump was in office.
A lot of that has to do with the lingering economic shock of the pandemic, and since 78% of Americans still `live paycheck to paycheck, Biden will still get blamed when people are confronted by absurd supermarket prices. Add to that interest rates are very high which makes it harder to afford new homes and cars. So all things said, despite the economy and stock market doing relatively well, people are still struggling which is why Trump is outpolling Biden on the economy, which is the number one issue that animates people to vote.