I did a study on this 25 years or so ago, and while there are always exceptions for an argument against the 60yr mandatory retirement from the flight deck, I found that overall the regulation works well.
I’m sure there are plenty of studies on both sides of the issue. The age 65 rule is already in effect in some countries and IATA has changed their guidelines. So I guess we’ll have to wait 3-5 years to see the results.