Give them an "Over 40 Starter Kit."
Give them a brutally honest "Life begins at 40" card.
Send them this Louis C.K. rant on how crappy it is to be 40.
Mention that babies born after they graduated from college are now in college themselves.
Gift them this "comprehensive" guide to sex after 40.
Throw an arm over their shoulder and say, "If you had a kid at 20, and then that kid had a baby at 20, you'd be a grandparent!"
Order them a subscription to AARP.
Help them battle the incontinence they're sure to experience now that they're of advanced age.
Or a mug that breaks down what kind of life they can expect from here on out.
If they love sports, you can say, "Did you realize you're now older than every player on the World Champion Boston Red Sox?"
Make them an adult diaper cake.
Give them an "over the hill" walker.
Say, "Forty, huh? If this were the 1700s you'd already be dead."
If the new 40-year-old is a man, give him a lifetime supply of "Viagra."
If it's a woman, give her something that will help her battle menopause.
Announce that you're relieved they'll never have a midlife crisis.
Present a gift that spells out exactly what they're older than.
Lastly, bake a cake that will help them mourn the loss of their youth.