Politicians are usually elderly here. Mostly it's because of connections, as well as a more comfortable predictability.
It's also typically when you have the largest abundance of assets, by large. Both in terms of financial ones, and in less tangible ones of relation, be it family or favors owed.
As to cognitive decline, well, I've not heard much beyond speculation on Trump's, but for Biden, it seems rather clear he was ignored because he was a convenient piece.
Perhaps even preferred, as it makes him more of a controllable puppet.