This is the 14993rd most frequent Russian word.
подростком
Teenager.
The word 'подростком' is used here as a noun in the instrumental case, describing the speaker's status as a teenager.
Когда я был подростком, мне нравилось читать книги.
When I was a teenager, I enjoyed reading books.
The word 'подростками' is the instrumental plural form, referring to the people the subject interacted with.
Ему нравилось гулять в парке с другими подростками.
He enjoyed walking in the park with other teenagers.
The word 'подростком' stands as a predicate nominative in the instrumental case, underlining the concept of existence in that state.