Also he's gotten too old. He's literally becoming dysfunctional at the role and I am sure knows it.
There's also no particular consequences for the party of him stepping down from leadership at this point -- it's not like he's going to lose a bunch of senior committee assignments for the GOP, for example. It isn't like with, say, Feinstein, where even though everyone thought she needed to go, they also knew that having her leave would be a disaster because the psychopaths across the aisle would refuse to let anyone else take her seats.
The consequence is that he is not a total simp for Trump the way most of the rest of the party is. Aid to Ukraine has been a very large division, to name one example.
I doubt age factored into his decision. He was too old ten years ago. If he had kept control of his party, he'd be clinging to power all the way to the morgue.