I took three reads to get Robin’s point, but it’s profound:

While the ratio of folks over 65 to younger adults (OADR) will almost
double in 45 years, the ratio of disabled to healthy adults (ADDR) will
hardly change at all.

In other words, as the fraction of retirees goes up, so will the fraction of able-bodied retirees.  In fact, we could maintain the status quo as long as people retired on the basis of how old they feel instead of how old they are.