How about some kind of trivia game featuring things from the year he/she was born?
Are you looking for an organized, everyone plays at the same time game? Pictionary or something like that featuring things from year of birth.
For more general thing--have a quiz featuring things from year of birth that everyone answers during the party and then at some point, some one tallies up the scores and reveals the answers.
This website will give you a lot of good info (popular songs, prices of gas and milk, historically significant events, etc): http://dmarie.com/timecap/
This sounds silly but it is a lot of fun! It is so hard to get the pins out and even funnier to watch people have handfuls of them and say they can't find any.
My cousin had a 40th party and actually had us play balloon pop. It sounds ridiculous for adults, but ended up being a lot of fun (especially after a few drinks). Two people go against each other and have to pop the balloon by sitting on it. You can't use your hands. The winner of each round faced off with each other until there was one winner. I laughed so hard during that game!
I can't imagine adults playing party games (like at a baby shower) for a 40th birthday, but in my family we would definitely play some beer pong and maybe some poker, setback or cribbage.