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
<----

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.

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.

All of them. :D

I do hope we are told the answer one day!

You are making a good point here. :D

I say the long hair one cause she got very little jaw and teens girls in game tend to have no jaw :D

I'm guessing these are all the same Sim that you've aged and dressed differently judging by the fact they all have the same ring. I was sure the middle one was an adult, at first, since she has the cheek lines adults get. But, we have the new cheek lines to add in CAS now, so you could've added those to fool us. The long-haired one is dressed the youngest, but again you could've done that to fool us. I picked the one with the bob hairstyle purely because it looks like she has the smoothest skin on her face.

It's very hard to tell the difference between the lifestages teen through adult, the changes are very subtle when they age up. I wouldn't be surprised if they were ALL actually teens, unless you didn't use the cheek lines on the Sim in the middle to make her look older. In that case, I'd guess she's an adult, the one on the left is a YA and the Sim on the right with the bob hairstyle is the teen.