[gambling8nt]

I don't agree with your choice of cutoff age--certainly it is correct that the historical figures who had made the largest contributions to the field of mathematics have done so while quite young--indeed, the prime ages for developing new mathematics seem historically to be between 17 and 23--but there is nothing to suggest predestination regarding the matter at age 5. Also, given the level of specialization involved with some open problems (and the overwhelmingly large amount of data involved in all of mathematics), it is certainly possible that someone deeply specializing in an aspect of mathematics not "popular" at the time could make a non-negligible contribution to mathematics even if they started late in life, and were less "smart" than their "prodigy" peers.

If you start mathematics late in life, you are extraordinarily unlikely to find your name on a list with names like Gauss, Newton, and Archimedes. Nor will you likely reach any sort of par with Euler, Hilbert, or Russell. Indeed, even the levels of Conway, Shannon, and Wiles are probably unattainable. But math is a huge field now, and there are parts of it that most young mathematicians find boring, or simply aren't exposed to. If you find some such part that you like, you can probably make some progress.