I am just curious to know why Bonnie Swanson, wife of Joe Swanson, was kept pregnant until the seventh season.
Because it's cartoons — Stewie is still a baby, too. Generally, cartoons are not too happy about altering the state of the show by letting big things happen.
Cartoons are for a lot of people not shows they follow consistently, rather they just watch a few episodes once in a while in no particular order. To that end, if the scenario changed in every episode (like it does with more continious story-telling tv shows like Breaking Bad), it would confuse the audience.
Imagine watching just a few random episodes of Breaking Bad: you'd have no idea what was going on. That's great for such a dramatic show, but not the norm in comedies and cartoons.
Family Guy, American Dad and shows like that are semi non-linear. They reference past seasons and so time does go on in their worlds, but they aren't that linear that they define age of characters.