I honestly believe a persons character, not age, determines how they will progress.
I have seen kids who have only been playing six or seven years, who are amazing, and others, who don't even know what a pentatonic is after playing even longer.
To me, I really think that what Alan says is bob on. It is what you practice that counts. I started playing in my late 20's, but in a way, that helped me. I always felt I was playing catch up, so my practice routines where very focussed and I never "played" during practice. I always said to myself that if I could learn a new lick a day, or improve on something or learn something that I couldn't do the day before, then the practice session was a success. If I couldn't, then I had failed, and I still stick to that idea now. It stll feels great now to look back even two months, and see progress.
I think adults are more likely to have that mentality than children. If I see a fantastic 18 year old guitar player, then I think he can only have been playing ten years or so, so why can I not achieve more in the same time?
"I like to play that guitar. I have to stare at it while I'm playing it because I'm not very good at playing it."
Noel Gallagher (who took the words right out of my mouth)