In answer to your question, if you take a look at table 3.3.6 in this pdf, you can surmise that about 44% would have had their first instance of abuse within about 5 years of ordination, about 61% within 10 years of ordination, and 74% within 15 years of ordination.
(that's assuming an average age of 28)
Hope that clarifies some.
That would strengthen the argument of a true decline.
Priests ordained in the eighties would have victims approaching and into the age ranges of the typical ‘reporting victim’
Thanks
Will Wallace