To me, adolescent means a young person of all ages (under age 18). Teenager is for adolescents who are aged 13-19. This is just coming from my own perspective as a native speaker

These words are synonyms where an adolescent is a more formal version of French origin, a teenager is from a basic stratum of vocabulary.

Adolescence- one's teenage years. Adolescent- a teenager. Just use "teenager" in everyday speech. "Adolescent" is an older word and a touch awkward.

"Adolescent" is an awkward word in everyday speech. Most people never use it. It's a good word to simply understand and not use much.

I believe its the stage between being an adult and a teenager. The transition from being a teen to being an adult.