"Some girls look twenty at fifteen."
Translation:Některé holky v patnácti vypadají na dvacet.
Not just US context, sounds like a paedophile's excuse It's a good sentence to learn the difference between na and v with vypadat, but the ages are so inappropriate.
Actually, it is very appropriate. People should be aware that appearances are often misleading.
I understand the concern expressed about this one, but it's a true statement, after all Some girls DO look twenty at fifteen, and some of them make a conscious decision to look older than they are.
Maybe, to avoid offending anyone and keeping the emphasis on the grammar, the sentence could be changed to "Some boys look twenty at fifteen" (or some other combination of ages)? That might be less controversial.