Actually, it's been upheld numerous times by the Supreme Court (the decider of what's Constitutional) that a mandatory retirement age is Constitutional when the issue is public safety.
You can throw out as many studies as you like that infer that flying at age 65 is as safe as 60, but there are just as many that show the opposite. Either way, your argument has been the pilots need to fly to 65 to recoup their losses financially. That is not sufficient reason to endanger the general population.
I bet if this age 60 thing changes, we see Boeings plowing thru terminals like Buicks plowing thru farmer's markets.