If the trainer is sensible, she'll start him easy, not go too high too fast, not push him more than he should go. I wouldn't trust him to the teenager alone, they always go too high too fast ;~D. He's in good athletic condition, he'll be fine.
UUUUH, I feel so bad. My husband on the phone talking to his brother about a horse. I heard him tell him, "I have to get me a horse. I feel like the stable boy" :'(
It's not a teenager, I think she's about 9-10.
Becky