The drinking age is 21 in Pennsylvania, and is strictly enforced in most bars in the city. If you look young, you will get carded everywhere, with the exception of a few complete dives (which will remain unnamed for their sake). Purchasing alcohol in Pennsylvania is not always simple. Beer distributors and state-run Wine & Spirits shops are located throughout the city and keep limited hours. See the Bring Your Own Bottle [1] page for more information.
Links:
[1] http://www.moon.com/destinations/philadelphia/restaurants/bring-your-own-bottle-0