Jokes about turning 40 usually put the age in perspective and find humor in creative ways of thinking about it. One joke is that starting smoking is fine at the age of 40, as it won't have enough time to hurt you.
A 40-year-old person may joke that he is 18 with 22 years of experience, indicating a young mindset despite the age. Alternatively, he may not consider 40 old at all, if he's a tree.
Helen Rowland once said that life began at 40. But so did fallen arches, rheumatism, faulty eyesight and the tendency to tell a story to the same person three or four times.