No one wants to hit the fifty mark. It's the age you technically stop being a young adult. Perhaps that has something to do with it.
The body really doesn't experience any noticeable changes at forty. At that age, all changes are in silent mode. You don't notice them right away, so you think you're one of the blessed few who seem to never age, that is--until you hit fifty.
There isn't a law or specific protocol that says don't actively pursue goals after reaching age fifty. Most of us have a built-in age sensor that shuts down success pursuits when we reach the targeted birthday.
Hating life after reaching a certain age is learned behavior or the result of a set of values given to us by cultural practice. This type of thinking is for the ordinary individual who follows the crowd over the cliff. Though we go through the motions of daily routine, every retiree has been heading toward that cliff of mediocrity. Whether we're fifteen or fifty, life is to be loved and lived--not hated and abandoned. Age amounts to knowledge and experience, and you need both to achieve all worthwhile endeavors.
