Honestly, according to me the age doesn't matter. What does matter is the intelligence, aptitude and skills that can be learned. Some learn fast, others learn slower.
The best moment for deciding to be a poker pro?
That would depend on a couple of things: can you beat the needed limit consistently? Do you have the discipline to play enough time and study to stay on top of the game? Do you love the game enough to stay playing when it becomes a job instead of a hobby?
That's the questions you should ask yourself, not if you have the right age...