You better believe you do! As of the date this article was written (March, 2011), you need a fishing license if you are under the age of 16 in Pennsylvania. Don't take this as legal advise though as things change and I don't want you to get in trouble. Here is a site to check out for the official rules of obtaining a license to fish - Click Here
You can obtain a state of PA fishing license at just about any sporting goods store that is a licensing issuing agent, county treasurer office and fish and boat commission office.
I found that the easiest way to fish legally in Pennsylvania is to purchase a license right online. It is really easy to do and you can print the license (and applicable trout, salmon, etc stamps) right on your printer.
I wrote an article on how to obtain a PA fishing license online. You can buy one right from the comforts of your printer. No driving to the store or waiting in line. The article is linked below:
How To Obtain PA Fishing License Buy Online Easy
If you get a chance, please also read some of my other TurtleDog posts.