PG-13, “for babies” and more The most difficult task when it comes to age appropriateness is finding something on TV or choosing a movie that will please a crowd of mixed-aged children. Babysitting advice: When you are in charge of what’s on TV or what movie gets picked for mixed-age children, err on the side of “babyish” instead of "PG-13."
In other words, the 12-year-old will rarely be interested in what the 5-year-old wants to watch, so instead of allowing Spider-Man 3 or 4 and scaring the 5-year-old into nightmares, go with Charlotte’s Web.
Tell the 12-year-old you know that this movie is babyish for him, but you appreciate his watching it for the sake of his little sister. Then observe: the 12-year-old will actually enjoy the movie, which in turn makes it age appropriate.
Games, puzzles and books If you’re concerned that a board game or a puzzle is either too difficult or too easy for a child, it won’t take long to find out. And in so doing, you don’t risk scaring the child, as you might with a television show or movie. As noted in the Babysitting Activities page, if you are babysitting any children under the age of seven or so, games and puzzles with tiny pieces are not a good idea. Often you'll find that younger ones want to play games their older siblings have played, which may turn into a pseudo-game that has very little to do with the playing the game the way it was intended.
Nick Jr.'s Web site has a fantastic page on appropriate games and activities for children through the age of six.
Books may be slightly more difficult to tell, but if you are reading to a child and you know there are words that the child isn't ready to either hear or understand—"stupid," "shut up," young-adult-type material—or vocabulary that’s so difficult that you are hardly able to get through a sentence without explaining what it all means, then try another book. Most likely the child will tell you he wants to read something else, even if he picked the book in the first place.