From season five, episode twelve, "May the Best Stan Win", it was revealed at the end of the episode exactly when Stan and Francine were born. Stan was born in 1967, and Francine in 1971. We see this information written on their gravestones when they died and are buried next to each other. Today, 2017, Stan is fifty-years-old and Francine is forty-six-years-old. Both will die in 2067, Stan will be one hundred-years-old, and Francine will be ninety-six-years-old.
I'm going over to the Juice Bar. You wouldn't like it, it's not about reliving childhood trauma it's all positive and about Juice.
In the Smith house there's a window at the stair landing that appears through most of season one, but is never seen again in subsequent seasons.
As of the 2008-2009 season, the intro sequence has been completely redone with more modern animation. There are a few minor changes to what happens during the sequence. For instance, Haley puts a peace sign post-it on Stan which Francine subsequently removes, and Klaus kicks Stan his car keys. The old "different newspaper" gag is removed, and instead each intro features Roger in the car with Stan, wearing a different outfit in each episode.
Up until the '08-'09 season, the opening credits featured a shot of Stan looking at a newspaper headline, which would be a joke headline relating to recent events. Episodes shown on Adult Swim have the headlines blanked out, leaving Stan looking at just white space. The credit sequence for new episodes for '08 - '09 eliminate this sequence altogether.