Except, with few exceptions, most of the teens in these shows are in their 20s. It has to do with child labor laws. Very few of the actors playing teens, if any, are actual teens. No one in Smallville was a teenager, they were all in their 20s. In Buffy? Only teens in that show - were well Buffy (Gellar was 17 or 18 at the beginning - playing 15/16, Dusku was 17, and Trachenberg 14 but everyone else was over 24). Beverly Hills 90210? Luke Perry was playing a teen at the age of 30.

Vampire Diaries? They were all in their twenties when they were in high school - it's why they pushed them out so fast.

Very few teen shows actually have teens cast in them.