How would you improve Teens?

Teen's Room Stuff Pack.

I also want a YA Room's SP. And an Adult Room's SP. And an Elder Room's SP.

So, more content.

Shorter teens is ridiculous and would break everything from item usage to CAS. I like that teens can wear anything in the CAS that the older Sims can. Provides more customization, compared to kids or toddlers, who are infinitely more limited. If you want some sort of 'obvious' way to spot a teen from a YA or older, I'd ask for a change in their name color or something simple like that.