I made a teen in CAS and now she is approaching YA. She got straight A's lvl 3 in her career and is in range for the responsible trait. She also got a boyfriend that their made promises to each other...
Townies start aging up from whenever they were spawned by the game, so he may or may not age up at the same time as you. If he doesn't age up at the same time you won't lose your romantic relationship progress (except the natural degrading of relationship that happens when you don't interact with them), you just won't be able to use any further romantic interactions until you're both aged to YA.