Nearly everyone shrinks with age. But some people insist, often after an annual visit to their doctor, that they’ve added a half-inch or so. If they aren’t children or teens, they’re probably mistaken, says Todd Milbrandt, a pediatric orthopedic surgeon at the Mayo Clinic in Rochester, Minn., who explains the significance of physes and what makes 20 a special number.

She spurts, she grows

All children grow at a slow rate until they stop, with spurts as infants and during adolescence. With a good diet rich in vitamin D and calcium, most girls will grow from age 10 to 14 and be completely done by 16, while boys grow from 12 until about 16 or 18, “with some, in rare circumstances, growing up until 20,” says Dr. Milbrandt, a board member of the Pediatric Orthopaedic Society of North America. After that, their growth plates, also called physes—the cartilage near the ends of each bone—are absorbed into the body, which forestalls further change.

“There may be a 21-year-old patient that is young, in terms of his bone age, which is why he may still be growing in college, whereas others may have stopped when they are 13 or 14,” says Dr. Milbrandt, who does research on growth plates.

Scientists are investigating what turns physes on and off, Dr. Milbrandt says. Pediatric endocrinologists might prescribe a hormone called IGF-1 to children of short stature during puberty to achieve average height. But it won’t work in adults “because adults do not have physes,” he says.

People with endocrine abnormalities can grow in height as adults, but they may have a condition called gigantism. “They usually have a pituitary tumor that produces too much of that growth hormone,” says Dr. Milbrandt. “But even they would grow only until 25.” Adults who keep growing past 21 or so should see a doctor, he says.
Have a tall morning

Some adults might mistakenly believe they have grown because height can vary during the day. In general, people are taller in the morning than in the afternoon, often because cells absorb more fluid overnight, Dr. Milbrandt says.

“Imagine a tire that is pumped up—that is high turgor,” he says. “When you wake up and you haven’t been standing all day, your turgor is at its peak, so you’re taller than you would be at night. You’re pumped up.” By the end of the day, gravity takes its toll and the spaces between the spinal disks have lost that turgor.

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