I'm sorry but why not raise the age to 70?
The 70 year old of today are as fit as the 50 year old of yesteryear, huge advances have been made in Society... no longer is 60 seen as 'old'.
Not all old fogeys nod off in the cruise
The age limit I suspect is purely economics with just a tad of Medical realism thrown in.
I know the Youngsters will be pissed off by this...