Exactly. Those who were abused in the 50s and before are older than 60, or dead, and the likelihood of a senior citizen coming forward now over a 50-year-old sex abuse is extremely remote.
Pederasty is endemic to the helping professions (teaching, coaching, counseling, ministry), and always has been.
The numbers the Herald ran were lifted directly from the AG's report and based on archdiocesan records of abuse reported at those times.
The report (though it doesn't seem all that worth reading) is at Attorney General's Report
An appendix, however, lists by year of graduation the numbers of accused priests who graduated from St. John's Seminary. The chart starts in 1946, so I'm assuming they didn't find any for the earlier 40's. To summarize by decade (see the AG's report, Appendix 2 for the chart):
1940s -- 5
1950s -- 33
1960s -- 52
1970s -- 16
1980s -- 5
1990s -- 2
1960 was, of course, the infamous class that produced both Geoghan and Shanley, if I recall, as well as the infamous Bishop McCormack.