That is true, though I think that you need to be a top junior to be able to compete in the men's tour, so I'd say the latest possible starting ages could be around 8-10 years old, so that you could compete in the U14 and U16 competitions later on. Feel and technique are so important in tennis and you learn those when you're young.
I didn't word my original post very well. Of course it's imperative that one starts young, but when I said it guarantees nothing, I meant that not everyone who starts young achieves success. For every top one-hundred player, there's ten players who get on tour but don't break it, and probably a thousand who never make it anyway.