When a child
 

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.

Re: When a child
 

I don't think grammatically from a prescriptivist point of view it would be correct but I could see someone actually saying that. They'd be omitting the understood part 'when (he was) a teenager.' But where I come from people omit quite a few things, but most noticeably 'I.'

Mark: What did you do yesterday?
My father: Went to the store. Bought some new shoes. Got home and went running. You?
Mark: Not much. Watched a little TV. Spoke to a friend on the phone and turned in early.