'when a child' vs 'when a teenager'

[Monica]

In the following fill-in-the-gap exercise: "He went back to the same school he had attended ... a teenager", I'm aware of the fact the most common answer would be "as a teenager", I was wondering whether "when a teenager" could be considered correct usage as well. Thank you.

[Jamie (K)]

Yes, "when a teenager" is possible and correct, but it would be unusual. "When a child" would be more familiar to people, but it doesn't mean the same thing.