There are "pensioner" Olympics , of a sort. Swimming and triathlon for instance have always had aged group competition world wide. Five year age groupings allow those of advanced years to continue to enjoy their sports till death takes them to a new start line. There is also Senior Grames, staged every four years just like the Olympics, with all the hubbub and striving for excellence that open competition have.
Like many of the experiences we can enjoy, it's about pitting yourself against your last best performance, much more than beating the next guy. Mature athletes definitely have heaps of fun. Just like disabled ones. Seen from a non athletes perspective this , like all sport, may seem like an odd way to spend lots of time and energy.
Taken instead , as a display of that incorrigable zeal humans have always had to pit themselves against the odds, to revel in their skills and energy, to exhult as only winners over personal limits can, age group and disabled competition makes the same sense as any other open sport. Long may it continue.