"How many centuries old now is this issue of clerical sexual abuse?"

It is credible that clergy pedophilia was known to exist by the 3rd century. Canon 71 of the Council of Elvira (around CE 306) says: "Men who sexually abuse boys shall not be given communion even at the end." Therefore, the issue of clerical sexual abuse is at least more than 17 centuries old.

http://www.awrsipe.com/patrick_wall/selected_documents/309%20council%20of%20elvira.pdf

The Church started to enforce celibacy since the 12th century so it is likely that the cover ups by Bishops of clergy pedophilia by priests started more than 800 years ago.