You do not want the decoration to appear childish as it would be embarrassing thing to have by adult. Of course, that does not mean you cannot have themed party if you want since you can still make everything appear mature if you want.
You can use any kind of theme that you want from fairytale, rustic, romance, and many others. What you need to do is to apply the theme in more mature way so it will appear beautiful and not childish.
Of course, you also want to make your birthday party to be fun thing to do. Thus, you can also combine any kind of colors that you love. Combining various colors will also make the table appear more beautiful to the eye.
If you want, you can use various items to decorate the table such as ribbon, lace, flowers, balloon, lights, and many others. Those items are very inexpensive so you can use them in anyway that you want without needing too much fund to have them all.
Try to use decoration that make the atmosphere feel youthful or you can try to use candle to create more romantic ambiance for the party. No matter which method that you want to use, just make sure that it is actually something that you love.
If you need to get some inspiration for your birthday party, then you might want to check out some of these table ornaments. These ornaments are very beautiful and will surely able to make the table appear more interesting. You can also try to make the ornament yourself if you have the time and skill.
image source : pinterest.com