Depends on the shop. Some shops are heavy into sales and a "professional" front end. For which they may not want a younger person for.
For just sitting in the back and fixing crap, I don't think age would matter much as long as you present yourself as confident, knowledgeable, and self-assured. In other words, act like a pro, but don't be ****y.
Most people are not impressed that you "built your own PC". If you have any formal training or book learning, use that as leverage.