@ksimmons93 No your not doing anything wrong I remember when I made Cassandra goth into a child she aged up as a teen and she was dressed completely different even her hair was different it's the same when you click on a young adult and change them to child or teen when you click on young adult their clothes are completely different.
Also forgot to add sometimes there face and body structure changes as well.
You're not doing it wrong, although I don't know how much they expected people to do that. (If it was an expected gameplay feature they would have tested it more.)
I don't know the mechanics of how the game ages up Sims and changes their features. I will say that Alexander Goth usually ages up fairly similar-looking; I think they optimized him to be extra-cute as a child so he always ages up with that really prominent nose, so... they clearly adjust the noses on age up. Sometimes the nose is slightly more... more than in other saves, but not by much. I think I only noticed because I play with the Goths so much.
Obviously, the game doesn't store hairstyles, outfits, etc. when you do this, so they'll all get RNG outfits and hair when they age back up. They'll also have different physical builds based on how you play them (if the kids do a lot of monkey bars and jungle gym they'll be more muscular and have less body fat; if they eat more they'll be heavier). There are also bug reports about how sometimes the age up tends to produce heavier Sims than you might expect. They should, however, have the same hair color, eye color, skin color, and features that are close to if not identical to the YA versions you aged down (barring weight related changes). Some hair colors are not available for toddlers or kids but become available at teen (I had Yuki Behr's children in one save and her kids often were blonde as toddlers and got one of the few teal hairstyles at age-up).
Anyway. In short, I don't think you're doing it wrong, but also the game doesn't store a template of their adult selves. I think their teen/adult features will be based on their child features and should be recognizable but may or may not be identical.
