Photo Booth photo cards are a great choice for milestone birthday invitations. There are spaces for four different photos, so you can include photos from years past. For someone turning 40, you could include a baby photo, a photo of the person as a teen, one as a young adult, and a current photo. Fun!
Shindigz sells so many milestone birthday party supplies. Milestone birthdays don't occur every day, so when one rolls around, make the celebration special with unique tableware, balloons, and invitations.