Recently-especially over the past thirty years-there have been significant changes in the way middle class American children play. Whereas children and adolescents used to spend a good deal of time engaged in unsupervised play and games, today sports are much more likely to be pursued through organized private sports leagues.
The entry of these organized leagues into the world of older children's play has changed the nature of that play. For one thing, these days, among younger adolescents, the level of play in sports such as basketball and soccer is noticeably higher than in the past. That's the result of an explosion in the amount of instruction and coaching available to aspiring players.
Although some of the adult involvement in these leagues is on a voluntary basis, for the most part club sports like volleyball or hockey cost money, often lots of money. Children's sports have entered the economy, and this has changed their character: they have become consumer goods and services that must be purchased. And often there is a genuine correlation between the product and the cost-to excel, children need good (often high priced) coaches, they need to travel nationwide to compete with other top athletes, etc.
One unfortunate consequence of these changes is that in many cases children are pushed too hard on the physical level, they are trained to use their bodies in ways that aren't developmentally appropriate. A striking example from Mark Hyman's book Until it Hurts: These days little league pitchers often learn to throw curve balls, a pitch that puts considerable strain on still-growing arms. That may be part of the reason that so few of the players who are stars in the Little League World Series ever make it to the major leagues. And baseball is by no means the only sport where these sorts of things are happening. In a number of youth sports it is increasingly common for children of high school and even middle school age to undergo major surgeries to address over-use injuries.