Is he a babysitter? Sometimes they won't age up when they're supposed to, and I've seen them skip the YA stage entirely. But if you invite them over, or any NPC, they usually show up in the correct age stage and stay there.
The -16 days until birthday likely means that the sim was supposed to age up 16 days ago, so yeah, he's stuck. You can use nraas MC to "trigger age transition" under sim>intermediate, but I'd do a MC reset on him first (under advanced).
There was a thread recently where someone said that a bunch of teens in the town were unable to age up. If I remember correctly, which I definitely might not, the culprit was some bad cc.