I hear this a lot, but I've never seen actual evidence of it. Everyone who started early that I've seen in computer science classes has been better than people who started later.
Caveat: we're both dealing purely anecdotal evidence and a fair bit of conjecture. We could easily be wrong.
But I've also spent a lot of time correcting bad habits, reading on good habits, and relearning a lot of things I did as a child -- and most of the code I wrote as a child was pretty awful.
Still, it's something that I've heard often and I can understand the logic behind it.