^Wrong^
Do you think BJ Penn is way better in BJJ now than when he was 5-8 years in? Or marcelo? Probably not. I dont think it would have mattered if he started at 16 or 25 either way he would have become great in under 5 years. I could be wrong but like anything the learning curve deceases with time in until eventually you peak and are pretty much as good as you ever will be.
Some guys that might be 5 years some 15 probably just depends alot on overall mat time and how fast of a learner you are. JMO