The younger characters on TV shows tend to age at an unusually fast pace so that they reach the teenage years and the more interesting storylines that come with them faster. A recent example is Marissa's younger sister on The OC - she was 10 when the show started 2 seasons ago, went off to boarding school, and is now 15 or 16 (and played by a different actress).
"There was this one time an alarm went off - it was in the middle of '24.' That's just wrong. Everyone knows everyone was watching '24' at that time."
--NYU student quoted in the Washington Square News about students ignoring the frequent fire alarms in dorms