What famous musicians from any time period had hits before they turned 18? Off the top of my head I can only think of Frankie Lymon, Shirley Temple, the Jacksons and the Osmonds…oh and all of those 80s Boy Bands. Thank you!
Paul Anka was 16 when he wrote and recorded his #1 hit “Diana.”
ETA: I’ve notified the Mods to move this thread to Cafe Society
Stevie Wonder, Steve Winwood and Alex Chilton come to mind.
Mozart.
Richie Valens?
Booker T Jones was still 17 when “Green Onions” hit the charts. Carl and Dennis Wilson were under 18 for the first few Beach Boys albums, and the David Marks who was with the band for 2 years was 14-15 during that span.
Hanson 
Tanya Tucker was 13 when she had her first hit, Delta Dawn.
Debbie Gibson and Tiffany (Darwish), both at about the same time. Gibson actually wrote her #1-charting song “Only in my Dreams”; Tiffany did cover versions (“I Think We’re Alone Now” and “I Saw Him Standing There”). Both were 16 when they launched their careers.
Billie Piper (later of Doctor Who fame) had a UK #1 at age 15, “Because We Want To” and followed up with another #1, “Girlfriend”.
Whatshername – that scrunchy-faced country chick who slept with the married guy…
LeAnn Rimes! The song was “Blue” and she was 13.
You laugh, but Justin Bieber is still only 17.
I also hesitate to mention the horrorshow that was Blazin’ Squad but I think they were all about 16 when they started. And what was the deal with Menudo?
Ritchie Valens was 17 when he died.
Judy Garland, certainly – principally a singer, though she made many movies.
Aaron Carter – say what you will about his talent, he’s a “professional” in the sense show business personalities use the term in praise of each other.
All three of the Hansens for their first album, and the two younger ones for the early part of their career. (I think a strong case can be made that they moved from boyband to competent serious musicians as they matured in their trade.)
Niggling at the back of my mind are several instrumentalists who started young, but no names come quickly to mind.
Vanessa Paradis was 14 when Joe le Taxi was a hit.
Doesn’t count for the OP due to her age at release, except for people to go “wow”, but Kate Bush was 19 when The Kick Inside was released, but she’d written most of the songs between the ages of 16 and 18.
Shirley Temple sold a half of million copies of the sheet music (yes sheet music!) at age 6 for “On the Good Ship Lollypop”. How much sheet music has Bieber sold?
Johnny Lang’s “Still Raining” hit #8 on the US charts when he was 17.
Moving to Cafe Society from GQ.
Colibri
General Questions Moderator
Rick Derringer was 17 when The McCoys hit #1 with “Hang On Sloopy.”
It depends on what you mean by “hit” but I think Taylor Swift qualifies.
I think all of Justin Bieber’s music has been sheet.