do your figures there not confuse "working age" with "in work"? And aren't we working age at 16 (so may still be in education)
Ultimately I agree with you though, pensions are essentially unsustainable. Final Salary is the most amazing and at the same time preposterous thing I'd ever heard
Generally speaking I think the ratio looks at things from a simple age perspective, as obviously some people carry on working past 65, just as some people of working age don't work. I suspect if you calculated it using those actually in work, it becomes even less attractive.