One funny joke for a 60th birthday is, "You know you're 60 when the candles on your cake set off the sprinkler system." Another one is, "When you hit 60, your cereal is chosen for the fiber content instead of the toy content."
Jokes for 60th birthdays are useful to lighten the mood and make the day a little easier for someone apprehensive about such a milestone year. People can tell jokes about the indignities and quirks of aging, but they should be good-natured and respect the recipient. A joke that is funny but also sensitive and light-hearted is, "You know you're 60 when you finally have to admit that your character lines are really just wrinkles."