If there's one thing I'll give Family Guy credit for, it's that they actually aged their characters... slowly over time, and mainly in the post-cancellation base breaking seasons, but Bonnie finally had the baby, I think they said Stewie was 2 somewhere (I can't remember), Chris moved on to High School, and Meg's 18. That last one actually brought about one for the few good episodes in recent history, where Quagmire was able to act on his creepy feelings for her, and those are the ONLY good ones she ever gets.
And the sad thing is, the kids (well, maybe not so much Stewie) get more mature than the freaking adults in that show, who constantly degrade into very nasty, immature jerks.
Meanwhile, as I said, Bart keeps going on summer vacation only to go back to 4th grade every year... ditto Arthur.