Only if they don't look like people in their early 20s.

The trope isn't about simple age discrepencies, but usually hovers around the 18 year old margin, or high school to early college range. The reason is that the actuality of a character playing the exact same age as themselves is rather slim, so we would have several million examples if some groundrules aren't layed down. One is that it should be at least a 3 year difference and the other is "teenger played by an adult" because of general Hollywood law on minors. Playing Hamlet and Playing Gertrude are the adult versions of this trope.