In the Commonwealth of Pennsylvania, the age of consent for sexual activity is 16. Therefore, anyone aged 16 and over can legally have sexual intercourse with an adult. If the complaintant is aged 13, 14, or 15, the defendant cannot be charged with statutory rape for having sexual intercourse with anyone that is less than four years older. Sexual intercourse with anyone age 12 or younger is automatically charged as statutory rape, regardless of the defendants age. Keep in mind that an 18 year old cannot be charged with statutory rape for having sex with a 14, 15, 16, or 17 year old but they
CAN be charged with corruption of a minor or unlawful contact, regardless whether or not the sexual intercourse was consentual.
http://wiki.answers.com/Q/What_are_the_statutory_rape_laws_in_the_state_of_PA
This makes it sound like you could get into some trouble, especially if her parents find out a 21 y/o man is banging their innocent 16 y/o minor daughter.
Before I got married, I always followed a simple formula for avoiding any "creepiness" of dating younger women: Your Age/2 + 7 = Min. Age to Date.