No law against dating. If consensual sex happens, the only potential crime that could be charged would be Corruption of Minors.

Can a 16 year old consent to a 20 year old in PA?

In Pennsylvania: People ages 16 and older can legally consent to sexual activity with anyone they choose, as long as the other person does not have authority over them as defined in Pennsylvanias institutional sexual assault statute.

Is it wrong for a 17 year old to date a 25 year old?

There is absolutely no illegality when a 25-year-old dates a 17-year-old.