A birthday party should be an enjoyable event for you and your guests, despite your age. Including humor in your invitations will help set the proper mood for your guests. As an adult, you can get away with more jokes than you could as a child, so don't be afraid to use a little mature humor if the occasion calls for it.
For many adults, aging is a sore spot. If your birthday boy has no qualms with his age, though, it's acceptable to poke gentle fun about it. Word your party invitations with a little sarcastic humor to lighten the mood. For example, you can include an anecdote like, "You are cordially invited to celebrate 'Old Man John's' 1000th birthday celebration." Including funny nicknames and a fake age can bring a smile to the invitation recipient's face.
When your party's guest list consists of close friends, it is acceptable to put an inside joke on your invitations. Consider your guest list and choose a humorous inside joke that everyone will recognize. Your joke may not make sense to people outside your close circle of friends, but that's not your concern. You can incorporate the joke into the wording of the invitation (for example, "Join Becky at her birthday bash for fun, drinks and cactus dancing") or include it separate from the invitation information in a separate story.
If your guest list consists of mature individuals with a slightly lewd sense of humor, you may consider putting a slightly off-color joke on your invitation. Keep it humorous, though, instead of downright dirty. You want to make your friends smile without making them uncomfortable. Only use mature humor on party invitations of close friends, though. Things can go bad quickly if you send an off-color invitation to a formal work acquaintance who is not familiar with your particular brand of humor.
Your friends and family share many memories with you, including some funny moments. For example, you can include a funny story from a birthday that took place in your childhood. The story should make your guests smile, but it can also teach them a little more about you. If you are writing the invitations for someone else's birthday, include a rather embarrassing story from the birthday girl's childhood. It adds another level of humor to the invitations and you can share a good laugh about it later when she sees the invitations for the first time.