I always use a lot of clothing for teens that look to fit their age range to look juvenile and casual. Graphic hoodies, graphic tops, sneakers, ripped/patched jeans, and colorful clothing. The knotted top with graphics on it from backyard stuff is perfect for teen girls.
I always make my teens a bit more round in the face. Add some details later when they age up depending. Though between idle animations specifically made for teens it's not too hard for me to identify them along with their voice. I still always give any teens a makeover.
Here's one of my teen boy sims. He's wearing a hoodie and a pair of jogger pants. Only gave him just a little bit of facial hair along with a stylish haircut. :p
https://i.imgur.com/D7Tb67m.png
And my Angela Pleasant I had made in game. Currently a teen. Simple dress for her, but I can tell she's a teen.
https://i.imgur.com/c4GO7Rs.png
