I stopped climbing for 20 years — marriage, kids, surfing — but then my daughter, Hannah, who was 11 at the time, asked to join the competitive youth team at Mission Cliffs, in San Francisco. I took her to sign up and felt a kind of future shock among the 50-foot walls and barbell racks, the yoga studio, spinning room and the pro shop. I hadn’t seen so many slender young men shirtless since high school swim practice, and I was particularly astonished to find nearly equal numbers of female climbers — lamentably scarce, in my day — chatting in big groups while friends swung on ropes overhead. The whole scene felt like an alcohol-free nightclub for cheerful, athletic nerds.

The truly unsettling part came a year later, when I discovered that Hannah and most of her skinny adolescent teammates routinely spidered up climbs harder than anything my friends and I did in our prime. These kids are fine athletes, but something else is going on here, too. Rock climbing, thanks in large part to the modern climbing gym, has suddenly started up the human performance curve toward athletic maturity.

Every other fundamental human movement pattern developed long ago into a mature sport. Competitive running dates back thousands of years, competitive swimming got started in the early 19th century, and both now enjoy such immense demographic reach that genetically suited outliers self-select for each and every subspecialty. Generations of accumulated training wisdom bring Olympians like the sprinter Usain Bolt or the distance swimmer Katie Ledecky to performances that have long since reached the outer limits of human potential. World records are broken by fractions of a second, and we now have an excellent sense of just how fast the fastest human beings can swim and run. As a result, a middle-aged father who ran a modest 4:50 mile at his last high school track meet is quite unlikely to see his daughter’s entire generation suddenly start running 3:45 miles, with a few kids clocking 3:30.

Something very like that, however, is happening to rock climbers of my generation. Thirty years ago, the sport drew mostly on the limited gene pool of outdoorsy families like mine, in New England, pockets of Appalachia, and the mountain West. Most people started as adults, training was ad hoc, and the actual climbing took place on summer weekends. In that context, I got the mistaken impression that I was a natural. But now all these climbing-gym birthday parties give vastly greater numbers of middle-class kids the chance to try climbing by the age of 10. If their parents can afford it (youth programs typically run from $100 to $200 a month), any kid with a knack can then join a team, training many times weekly, and compete on a circuit of nearly 200 annual climbing competitions nationwide.

As a result, every subcategory of climbing now selects girls and boys of just the right body types and personalities, trains them hard during formative years when the human body works like a neurological sponge, and then sets them loose to shatter outdoor performance standards. Sasha DiGiulian, Daniel Woods, Chris Sharma — every one of these rock-climbing superstars developed superhuman finger strength and body control in climbing gyms. Same for Alex Honnold, who combines extraordinary physical gifts with truly unusual psychological architecture: He started climbing as a child in a gym in Sacramento; found his way to free-soloing, in which he climbs sheer cliffs alone without ropes; and is now much better than anyone dreamed possible 10 years ago. Then there’s Ashima Shiraishi, the tiny Manhattan gym climber who, at age 14, is a serious contender to become the greatest living climber of either sex.