I feel like at this point, age is really the only way to attack Biden, which he can't defend against. When he entered the office, it was still the pandemic, and the economy took a hit. Inflation seems to be settling down now, and it seems like the Fed may get away with the soft landing. In a year, the economy could be turning around, who knows. The Republicans want the Hunter Biden relationship to show that he's corrupt like Trump, but there's nothing there, and that will fizzle out before election day.
There's tons of other things to attack Biden on, but in most cases, it's an issue he inherited, something which might not be an issue in a year, or general Republicans inaction to make him look bad. Age is easy to attack because he can't be younger, and somehow voters are too dumb to realize Trump is just two years younger than Biden. Literally every sentence about age and physical/mental health applies to Trump as well.