One joke for people turning 50 is that they can stop calling their wrinkles "laugh lines" because nothing is that funny. Another joke is that by age 50, people's wild oats have turned to porridge.
Many jokes for people turning 50 are related to changes in the body. For example, women know they're turning 50 when going without their bras pulls the wrinkles out of their faces. Another joke is that "Victoria's Secret" actually is the secret that no one over 50 can fit into a thong.
Another joke is a little prayer that 50-year-olds can have the senility to forget those they don't like, the good luck to run into those they do like and the eyesight to tell the difference.
Some jokes relate to the change in mental faculties caused by aging. For example, one joke goes that a 50-year-old man observes that a light goes on every time he has to get up in the middle of the night to urinate. His wife points out that means he's urinating in the refrigerator again.
Another joke is a bit of advice for befuddled 50-year-olds. If they ever get lost, they should just change where they wanted to go. Likewise, a joke observes that people who haven't grown up by the age of 50 don't have to.