strange that they're all retired
.
I largely agree in principle however that 65 should be the upper age limit for
running for office for a first term and if going for a 2nd term over 65, it would have to be immediately following the first. That would put an effective "age limit" on an active president of 73. Which I think is quite a generous upper limit, really.
In the UK it's different since we don't elect a PM. I think a sensible age limit for leaving the PM position (and TBH the front benches) would be 70, barring there being extenuating circumstances for staying on (e.g. global crisis or national emergency).
It may seem "ageist" but to be honest, it's very UNusual for someone pushing their 80's to not be losing some mental alacrity. That doesn't mean they're "senile" or "confused" but it's a biological inevitability that people start slowing down a bit. Some people are very lucky and stay very sharp but at 80+ it's certainly not the majority.