Is it possible for a teen to take an adult to the prom?

The short answer is no.

If it is some consolation, without any knowledge of this, in nature, impossible mission, one of teen sims managed to have a bf of hers on the prom. That was probably just a glitch (he had a birthday a day before) but now I started to wonder if she actually smuggled him through the window or fence, or he just looked so innocent or... he just missed these last classes so he was still considered as "a late student". No Idea.

Just mysteries of The Sims.