If you go to any park or recreational center, you are likely to see a lot of young people enjoying themselves. Kids know how to play. It is not something they need to be taught to do normally.
It does not matter if their activities are structured playing a game with formal rules. Kids will simply find a way to have fun. However, when kids grow up and turn into adults their lives change. They often do not have the time to have a lot of fun. They must go to work. They must raise their children. They have other responsibilities.
When they retire, they have the time to have fun again. However, it is somewhat rare to see a group of elderly people at a park or rec center having fun. They have forgotten what they once knew how to do, as the Wills, Trusts & Estates Prof Blog discusses in "A Lost Skill Among the Elderly: How to Have Fun."
What this means for retired people is that a lot of them spend a lot of time watching television for hours and hours at a time. While some television viewing is not going to hurt anyone, research suggests that elderly people who are mentally and physically active see health benefits. It would be better for elderly people to spend less time in front of the television and more time having fun.
Of course, not all elderly people are physically capable of playing the same games they did as a child. However, there are other activities available for the elderly in most places.
Reference: Wills, Trusts & Estates Prof Blog (June 13, 2018) "A Lost Skill Among the Elderly: How to Have Fun."