Ah right, I see that reading now. I didn't mean that working hours is proportional to age; just that you work "proportionally less" pre-65 in the sense that you work more post-65 than the current norm of "none", and then reallocate those extra person-years of productivity to reduce the pre-65 work week.
Maybe wanting to stop working at 65 and step into full retirement is particular to the American system. Most 70 year olds I know would like some kind of job/retirement hybrid: work 30 hours a week, have longer vacation times, more flexible hours, not losing the job when you have to take a leave for medical reasons, etc.