That depends. Retirement as passage into idleness? Sure. Many people who retire piss away the remainder of their lives because they don't know what to do with themselves. They get depressed, and they become lonely, because they become isolated. Children move away, spouses die, friends move away or die. Couple this with suburban planning where you can't do anything without a car (something the elderly are less likely to be able to operate) and where there is no town center, certainly not one congenial to the social life of the neighborhood or town. With the decline in church membership, you don't have that option anymore, one that also dealt with questions dealing with the end of life (if you view death as the absolute end, then your life was hopeless to begin with; retirement and old age simply strip distractions and illusions effectively).

But this is separate from the question of retirement as such, or the question of aging. Your grandfather, I presume, only refused to take medication and medical treatment, which falls under "extraordinary care", and this is morally licit. Had he refused food or offed himself intentionally (or with "assistance"), that would have been a different matter.