You are assuming that as your age increases you eventually become more and more debilitated. Although this has been true for all of human history, it's not unreasonable to think that this might change during our lifetime. See SENS:
People associate frailty with age, but it doesn't have to be that way. If we can fix or clean up the underlying processes that cause aging, being 90 years old will be a very different experience.
http://www.sens.org/
People associate frailty with age, but it doesn't have to be that way. If we can fix or clean up the underlying processes that cause aging, being 90 years old will be a very different experience.