I don’t disagree on your last point, but age is a factor that I don’t think matters as much as you imply at first. In the sense that different people are affected in different ways. A good president or politician won’t be defined by their age. You know who is older than Biden? Sanders, and he might be one of the most lucid and seemingly good politicians all around (but I’m not in the USA so my understanding is limited, at least what I have heard of him is extremely positive).
That’s not to say that I’d rather have younger people involved in important roles in politics, there’s too many that should be completely retired, and Biden is one of them. But age is not the main issue.
Generic statements such as “X age is too old for anyone to be president” is the issue. Some people at 80 could be healthy and lucid enough to be president and some at 60 might not. Age is not a limiting factor equally for everyone, so such judging should be made individually for each person and not with a blank statement.
Not accountable to death is a complex topic, for the same reason that they won’t see the consequences of their ruling, they might also rule for more “pure” reasons and not to make friends in high places or pocket all they can to retire and live life. So I’m not sure that’s the negative point you seem to think it is.
The fact that someone is more likely to die soon shouldn’t be a reason to block them from doing something long term, or think they can’t provide a real positive effect.
What we do agree on is that the lack of younger representation in politics is indeed a worrying topic.