Which one is the Teenager?

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.