1. In the UK, if the Head of State (who has minimal political importance) can be replaced by a regent, which under current law would not be a problem. The Head of Government not only tend to be younger (than American presidents) but are very easy to replace (since, in theory, they serve at the pleasure of the monarch, so with approval of the majority party, they can be replaced if disabled).
2. The United States combined Head of State with Head of Government, and we have no easy way of replacing a disabled president (other than with the vice-president, which itself raises problems. There is no provision for a special election. And the US tends to elect only someone who is well established (and therefore old). And since American president is Head of Government, he has manage the government on a day to day basis. So it is reasonable that many Americans are concerned that both parties seem to be oriented towards nominating presidential candidates who are well past their “sell by” dates.