We want the stability of someone who has seen life’s ups and downs and understands there is more to life than the petty day to days of a presidency. There is a legacy beyond them and they imbue that in their policy.
Age can be a symptom of your inability to do this, but it is not the problem.
The problem is with specific mental ailments and behavior coming into the presidency. We should scrutinize those ailments heavily, and build a culture around stepping down when life inevitably gets to you too — and having it taken from you if you do not take the opportunity for mutual dignity.
The problems that afflicted Biden could happen to anyone at any age. It is a problem if any candidate experiences it.
And I do not really think age has effectively changed Trump’s view of the world.
I'd absolutely support a maximum age limit, maybe e.g. if you will reach 70 years of age in your term of office you cannot run. So a senator could be elected up to age 64. A Representative up to age 68. And I'd apply that to all elected offices. My biggest criticism of Biden vs. Trump was that they were both too old.