Don’t know for sure, but I can tell you what a friend of mine was told by his guitar teacher. The guy in question is my bass player’s brother. The bass player and I played in bands from when we were kids. The brother decided to learn guitar when he was in his thirties–mid-thirties, I think. Never played anything before.
Anyway, his teacher told him the best he could make him was competent, but he’d never be a strong guitarist, a real player. The teacher’s theory is that you become hard-wired by a certain age, and your mental wiring fights you to learn things like musical instruments. You’re already wired to not know how to play. According to the teacher, a kid is easier. Not every kid will be a good musician, but all kids are blank programs, susceptible to influence and instruction.
Have no idea if this holds water, but it seems intriguing. My bass player is a terrific musician, and a fine singer. That was our forte–we had strong harmonies in our band, and everybody could play. Same guy’s brother can strum a few chords (at this point anyway) and not much more.