Which age-gates should be skill-gates and vice-versa?

It's an interesting thought, but the reason voting doesn't have a 'skill gate' is because it's seen as an easy way for authoritarian governments/forces to bar those they dislike/those that disagree with them from voting. Think back to how those 'tests' they made people do in the US in the Jim Crow era.

At the same time, it's pretty clear the usual age restrictions on voting feel about as arbitrary. That a lot of the time, those above the minimum aren't necessarily any more mature or capable of reason than those below it.

Not sure about driving though. Given there's already a skill test involved there, I suspect the age part is more a question of practicality and potential safety concerns than anything else.