The teenager has modified each of the bikes to go faster than he is legally allowed to ride them; in fact, the Talaria can hit 70 miles per hour. His mother gave him her blessing, she said, and even helped him clip a wire that removes the speed “governor” that ordinarily limits the vehicle to 20 miles per hour.

She posited that the companies designed the bikes to allow the speed caps to be removed. “They want you to be in charge of doing it,” she said, “because they don’t want to be held liable producing a bike that goes 55 miles per hour where a kid goes straight into the concrete.”

Gari Hewitt, a nurse in the area and a friend of the mother’s, expressed more caution about e-bikes. Not long ago, she saw a 12-year-old boy lying unconscious in the street. He had been riding a Super73 when he hit a rock and “flew over the handlebars,” said Ms. Hewitt, who works as a nurse in a pediatric trauma unit. She checked out the boy before he was sent to the hospital; she later learned that he had a punctured lung, among other injuries.

Ms. Hewitt has two teenagers of her own, a 15-year-old girl and a 14-year-old boy. Each received an e-bike for Christmas. “When they’re this age, how do you wow them?” Ms. Hewitt asked. “We only have a couple of years left to wow them.”

The e-bikes came with rules: Always wear a helmet, don’t exceed 20 miles an hour, never ride at night. The hospital where she works considers any crash at speeds of 20 miles per hour or greater to be “a trauma activation,” she said.

“But you could hurt yourself on a bike, too,” she said. “Everything comes with responsibility.”