I admit, I kind of like that headcanon about Stiles being held back for whatever reason. It even gave Scott a little mention.

On the Derek point. If he was over 25 the hanging around the High school point would be one of the least creepiest things given two of his scenes with Erica.

Someone did question it a little while back, whether Jeff meant he was older than people thought because they thought he was early 20s. Or whether he said that based on his supposed age in the pilot, which was 19.