In Pennsylvania, the age of consent is 16.
But the 16 year old can not just have sex with anyone, the person they have sex with must not be more than 4 years older than the 16 year old or the older person will face statutory rape charges.
While the previous answer concerning consent and age differences for the purpose of statutory sexual assault may be accurate, the answer to your question does not end there.
In PA, the charge of corruption of minors includes sexual acts engaged in between two people where one is under 18 and the other over 18. This offense is a felony.
Additionally, in December of 2012, amendments were made to Megan's Law in PA, which is the law that defines who is required to register as a sex offender and for how long upon conviction for sex offenses. This amendment was made to bring PA in compliance with SORNA (Sex Offender Registration and Notification Act). As a result of these amendments, a person convicted of Corruption of Minors of a sexual nature will be required to register as a sex offender for 15 years at a minimum.
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A similar question was asked just a few days ago with excellent answers by my three of my colleagues. I strongly encourage you to scroll back and read their answers.
However, to directly answer your question, the 20 year old adult who has sex with a 17 year old can be charged with corruption of minors; a felony of the third degree.
I think I answered this before. legally yes, but if this is the case where her father is angry at you, you should stay away. I have represented far too many guys who had consensual sex with young women at first, but when the boyfriend or father or brother finds out, rape charges are filed and all of the sudden you are sitting before a jury in your best Sunday suit.