Actors on Disney channel

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is it just me or does disney make their actors seem very young? Like for example the disney channel show "Sonny with a chance" Chad Dylan Cooperis like 17 in the show but he is played by a 21 year old actor called terling knight. Okay this might not be a good example since the age gap isn't that big but let's take another example. In Hannah Montanna, miley's big bro is like 17-18 in the show but in real life the actor is like 33 or something, it,s just crazy how they make their actors seem so young :P don't you think?