The vet was completely double-booked yesterday. That is the longest my birds have ever spent at their office. I'm afraid my follow-up detailed conversation won't happen until today--perhaps even as an email exchange. So, I haven't gotten an age estimate from him yet.
That's my guess, too. He is small as well as thin. He was in a flight cage with at least 6 other budgies, and there were some adult hens in that cage. I mention that because I imagine some sassy-pants little hen didn't like the looks of him, poor lad!