The reason our leaders suck is not because they're old, but because democracy selects mostly for charisma, showmanship, and if we're even more cynical - a lack of morals/ethics. Political careers are most expedited by telling people what they want to hear, and by selling yourself out to the highest bidder so you can get funded by corporations and other moneyed interests. A great conman would likely make a great democratic politician. A typical definition of leadership is the four Cs - character, competence, commitment, and courage. The democratic political system doesn't select for that, at all. We want e.g. generals who rose on merit, instead we get lawyers.

As for age itself, there's also a pretty straight forward reason that we tend to pick older leaders. As people age they're generally going to find their younger selves less than desirable - naivete, gullibility, ignorance, and just generally a lack of wisdom. So I'd be terrified of having a leader in e.g. their 30s simply because I don't think they'd have anywhere near the life experience to have accumulated a reasonable base of wisdom. And I suspect as I continue to age, that 30 is going to get pushed higher and higher. Even back in Ancient Rome one could not run for Consul until you were at least 42 years old. And people likely 'grew up' much, much, faster in those times than we do now a days, simply because life was so much more testing.

I don't think it matters one way or the other how old leaders are. If they are capable, put them in charge. If not, don't.