Adult Birthday Cakes Can BE adult, or they can be for adults. It is up to you to decide where your pastries cross the line.
There is no need to have a boring, generic birthday cake just because you are turning twenty nine (again). Why should the little kids get all of the colorful themes and super adorable birthday cakes? Don't the adults get a little bit of cute in butter cream too?
As an adult, it should be your age given right to demand the flavor combinations that you would like. If you would like chocolate fudge, Bavarian cream filled, butter cream frosted cake, or another special cake filling then so be it.
An adult birthday cake theme should reflect the adult that is just quivering with desire to lap icing off the knife. Sports fans should have the sport theme or color of their favorite team. Movie fans should have something related to their favorite movie. And only the gardener among us should have sugar roses, ever.
If you are having a small gathering of your favorite friends and you are a little more flamboyant, than you might enjoy a more "For Adults Only" themed birthday cake. There are many shops that make cakes that can range from PG 13 to Oh Molly!, so choose according to your recipient.