I take a tangent to talk not about what age, but about what minds. Many smart people can solve all kinds of problems but still do poorly in PhD, most dropping out. Why? Because part of PhD is to find your problem, and cut it from open-ended to specifically scoped. Many smart people can solve all kinds of problems, but the problems have to be given to them. (Of course, finding your problem is not enough, you still have to solve it. But you already know this.) Some thesis advisors can suggest pretty specific problems; some schools sometimes actually advertise "PhD position: such-and-such specific project". If you run into one of those, good for you, someone is giving the problem to you, you're like half-done. But this is the minority. The majority is more like: the thesis advisor is too helpful and too open, he/she suggests too many problems and too many variations, so you're none the wiser. :)