18 Pa.C.S.A. §3102.  Mistake as to age.

Except as otherwise provided, whenever in this chapter the criminality of conduct depends on a child being below the age of 14 years, it is no defense that the defendant did not know the age of the child or reasonably believed the child to be the age of 14 years or older. When criminality depends on the child’s being below a critical age older than 14 years, it is a defense for the defendant to prove by a preponderance of the evidence that he or she reasonably believed the child to be above the critical age.