Males

• Novelty neckties always prove to be fun gag gifts for any occasion, and there's no need to leave 50th birthday off that list. A tie with a witty statement on it or even a tie with a skeleton sitting in front of a birthday cake and a party hat on can definitely send the message that the birthday boy is getting older. Along the same lines, you can also give a ball cap with a phrase on it that may stand out. "Old Fart" is a favorite, but an uncommon hat is one that displays a want ad stating, "50 year old, one owner, needs parts, make offer."
  
  

Females

• T-shirts are a great medium for conveying someone's age and how they feel about it, or better yet, how a gift-giver feels about it. There are tees that state how good the wearer looks, such as "It took me 50 years to look this good." Alternatively, you can give a shirt that pokes fun at the age, such as "Older than the dirt in my garden." Buttons are a gift that can display similar messages, "Older than dirt and twice as dirty."
  
  

Unisex

• Coffee mugs have been a birthday staple among 50-year-olds for decades. These can display messages such as "Celebrating the 32nd anniversary of my 18th birthday." You can buy a button that says "Shhh, nobody knows I'm turning 50." Finally, just buying products that may actually be useful, but are associated with old age, will typically get a laugh. Some examples are adult diapers, arthritis soothing cream and denture cream.
  
  

Homemade

• There's a classic sentimental gift that can be played for both a good laugh and a bit of sentiment as well. Take newspaper clippings, magazine articles and advertisements from as close to the day the recipient was born -- this will be the sentimental portion. The gag comes in when the recipient realizes that you've also added a few clippings from 50 years prior to even that, or homemade clippings referring to dinosaurs and cavemen. A "bouquet" of 50 lollipops tied with a ribbon that says "50 sucks" on it will almost certainly produce a chuckle.