One-liners and short jokes are the ones that are best. It is obvious that most fortieth birthday jokes will end with a great punch line about becoming older. If your joke is too long, then it could get boring and end up with the listener anticipating the typical punch line before you can get to it. Therefore short, quick jokes are the answer.
You can try and be original by making up your own fortieth birthday jokes. This can be done by changing around certain details so that they actually relate to personal situations in the person's life. Mix personal stories about the "birthday boy/gal" with your jokes so that they are more entertaining. All the time, you have to remember that the main aim of all these jokes is to have fun - so do not go overboard and be insulting and sarcastic etc.