But when do children become adults? It's a gradual thing.
If someone asks "Can I borrow your shotgun?" and you can say "yes" without misgivings, you're dealing with an adult
I suppose in music education, I think of as an adult as somebody who is willing to take complete responsibility for his or her musical development. I'm sure it's gradual, but I suspect you know when it's happened. This is what I observed in my son, anyway. I couldn't tell you when he changed from a musical child to a musical adult, but I can see that he has. Unfortunately for him, he isn't actually an adult in the chronological sense, and I'm still the one paying his teachers, so he doesn't have the complete autonomy that he would prefer.
I think if you have a strong interest in music as a child, and a natural "good ear", as I had, it's all too easy to end up where I am today -- unable to play music that doesn't match the tastes you had as a teenager. On the other hand, I worry that forcing a child into conventional music teaching at a young age runs the risk of stifling the natural interest.
I had one piano lesson when I was about seven years old. But since I could already play everything I heard by ear, why would have I been interested in Hot Cross Buns? If somebody had told me that, when I was 40, I would regret not taking an interest in Hot Cross Buns, I wouldn't have been overly concerned.
My son went through the Hot Cross Buns stage at age seven, and came out unscathed, happily. However, I encouraged him to play what he really wanted to play as well, and helped him to do so. I had to work hard to keep him practicing scales and so on, but there was never any problem getting him to play movie themes by ear.
These days, because he's a musical adult, he practices scales and exercises himself because he's worked out that he needs to, not because anybody is telling him to. I'm still not sure I'd lend him my shotgun, however.