Aging in comic strips is always a problem -- usually it's one that's simply ignored. Dennis the Menace will always be 5 going on 6. Garfield the comic strip has run longer then the natural lifespan of a cat (even one with good eating habits). And so one.
A few strips let their characters age, such as the first iteration of For Better or Worse, or Gasoline Alley (see: For Better or Worse - the Aging Process). Usually, though, the creators make a choice. But not every creator isn't Francesco Marciuliano, who's had fun with comics conventions before. (see my previous posts about Sally Forth)
Pay close attention to the second panel in this September 14, 2014 sequence of Sally Forth. (click on image to enlarge)
One child ages, while the other remains 12. Best not to look at that too closely, or indeed, the whole system will fall apart. But for the observant reader, Ted's warning provides some additional humor.