Which one is the Teenager?

I put long hair, but I am not meaning the one with the hair worn up. So perhaps I should have picked side swept hair, but it is the only one that looks long to me.

I guess
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but actually it could be any of them cause ts4 does not do good job at making teens different from adult

The one on the left... I wasn't sure if that was long hair or side-swept.

Hard to tell if it's girl, it's easier if it's a boy but I believe the left one has more youthful look on her.

Long hair. Teens' heads are slightly bigger and their bodies are very slender.

This is a thread on point.
But there isn't much difference between YA and Adult either, and Elder looks like 60 year olds (but behave as bitter 90's). It's one of the reasons it makes me think the game is directed to 12 years olds. I was also very surprised when I first started to read the TS3 section, and how many there was that chose the teenager state as a sim to play with. I first tried the game in adult age, but I don't think that is very common.

I think the long-haired one, she seems to have puffier cheeks compared to the others.

All of them. :D

You can't really tell. Teens are the same height as young adults, adults, and elders.

I do hope we are told the answer one day!