Why did you use who instead of to?

Andrew is totally correct. h However, the original sentence is more elegant and refined than the sentence with "who" . In academic English, you will find this phrasing over the sentence with "who." the only (noun) to (do something) She was the only child to come up with that answer. (academic English, or "newspaper" English) She was the only child who came up with that answer. (Spoken or less formal English)

"He is not the only teenager who takes this kind of risk." That would be the correct way to say the sentence with "who" instead of "to."