Your average sports fan is an athlete in their own right and just like the pros there comes a time when you have to shut it down. This is what the typical life cycle of a recreational athlete looks like.
18-25: You're fresh out of high school and in the best shape of your life. You routinely play pick up games at the park or a university intramural facility. You can play 12-15 games in a row on a regulation size court and 8-10 softball games in a weekend. You push the younger guys around and you crush the older guys. All you need is a tall glass of orange juice and you can go all day.
25-30: You settle into work and family life, but the competitive fire still burns. You join rec leagues and participate in 3 on 3 tournaments when your schedule permits. The regulation size court looks a little bigger than you remember and that 12-15 games is more like 8-10 games. You feel a slight twinge or pull here and there, but you ignore it because you've never had a problem in the past and you like to feel invincible. On the softball circuit you start platooning with another player and only make half the games.
30-35: For those who can still play regulation courts are off limits unless you want your lungs to collapse and burst into flames. You got 5 games in you at most. The young guys torch you and you realize you have no advantage over the old guys whatsoever. Your brain can't wrap itself around the concept that you are the old guy so you push your body to do things it can't do anymore. Casts, crutches, and knee braces become common place amongst your friends.
35 and beyond: You and your friends exchange war stories about various injuries and you wonder if it's even worth it to get back on the field or court. You watch older players on television (Ray Allen, Derek Fisher, Jim Thome) and you get inspired to give it one more try. Your body goes snap, crackle, pop like a rice krispies commercial and you end up on the sidelines permanently.
The better the athlete, the longer this cycle goes. The better your memory of past glory, the more it will betray you. If this cycle sounds familiar always remember the swimming pool is your friend. Less wear and tear on the body and no ego to bruise.