(Apocryphally) Randy California would have been in the Jimi Hendrix Experience at age 15 if his parents had let him go to England. Instead he joined Spirit at 16 and wrote “I Got A Line On You” for their second album (still at or before his eighteenth year).
Although he’s never had a “hit” (a telling commentary on the lack of taste of the music-buying public), Derek Trucks was drawing attention at age 9, sitting in with lots of serious players, and forming his own band at age 15 (but not releasing anything under that name until 1997. Derek was born in June of 1979. I’ll leave it to you to determine if he qualifies.
Roine Stolt joined Kaipa at age 17. (No American hits, but then, it is prog.)
Another early bloomer was Eric Johnson. He had no hits before the 18 year cutoff, but had joined a well respected regional band (Mariani) by 14 or 15, and another, electromagnets shortly after. No hits from either of them, but each recorded a highly collectible demo (The Electomagnets’ would be re-released decades later). He would be well past 18 before he had his first “hit”, but had recorded a LOT of good music by then.