I tried to do different things with the teenagers to make them feel different, but I know what you mean, they need more life and a discovery of life. I've built arenas, arcades, cafes, all in an effort to have more things for my teens to do and try to re-create that and it really didn't work. It needs the proper interactions/reactions to go with them. Sure the stuff I built was fun, but YA-Elders can have fun there too, even children in a lot of cases although I did these buildings with teens in mind. It didn't fix the problem though. The teens need some stuff for just them to make them feel different from other stages and they should look different too. It bugs me too sometimes if my sim is at a park and chatting it up and I think they're making a friend and I see it's a teen then just feel kinda creepy... Part of why some of this is missing I feel like is because it's missing for adults. No sims have likes or dislikes or favourites or anything. The teen problem is part of a much larger core game problem.
All the life stages need work though to be honest. (Except for Toddlers, YA and Adult). Elders and children are neglected and I would like more interactions for them. I remember my TS3 elders whipping out wallet photos to show off their family to other sims, that was so sweet! I'd like my elders to feel more distinguished, I feel like they've lived long enough to deserve it. Children should be able to interact with each other more and act like children. Pillow fights, playing pretend, tag. All my sim children want to do anymore is be alone with a tablet. If I take electronics out of the house, they talk more to each other, but that's it. It's not their fault though (the sim kids), the interactions simply aren't there.