Some great 40th birthday jokes are as follows: "Forty years old? You’re 4 perfect 10's!" "No woman should have kids over 40. Really, 40 kids is enough" and "Feeling over the hill at 40? No worries. Popping bubble wrap turns everybody into a little kid again."
Some other great jokes to tell a loved one or friend who is turning 40 are: "You're not getting older, you're increasing in value" and "Forty and aging like fine wine." When cracking an age joke, it is always good to act like the subject of the joke is younger than she actually is. For example, "You're not 40, you're 18 with 22 years of experience."
If you wish to give a more "tangible" laugh, a trip to a local drug store or party store might wield some hilarious cheap 40th birthday cards. Plays on dates can also be used. If the subject of the jokes is turning 40 in 2015, shirts that say "Established in 1975" can be made.
"Forty ain't for the weak" "Fourteen thousand, six hundred days old, but who's counting?" "Forty and fabulous" "Forty is the new 30" and "Forty is only 11 in Scrabble" are all great jokes to crack at a 40th birthday party.