I got inspired after watching America's Got Talent the other day, with a 9-year old guitarist who knew who Stevie Ray Vaughan was played. So what age is the youngest actual talented musician you know or have ever seen? Playing an instrument, not singing.
I saw a musical savant when I was 14. The kid had autism, but he was a crazy good piano player. He was 6. I also personally know a guy who covered Dream Theater's Erotomania on guitar. He was 11 when he did that.