Rachael Freedland had plenty of doubts when she joined New York City’s Empire Triathlon Club three years ago at age 35. She had long wanted to compete in the multisport races, but at that point, she could not run a mile, had barely swum since childhood and did not feel ready to invest in a fancy, lightweight racing bicycle like the ones many of her fellow competitors would be using.

To Alison Kreideweis, the Empire club’s founder, Freedland looked like an ideal recruit.

After years of explosive growth in the United States, triathlon participation had entered a steep decline, its appeal diminished among younger people — in part, industry observers say, because the increasing sophistication of equipment had driven up costs.

So Kreideweis, in line with the sport’s increasing emphasis on promoting the races as doable, told Freedland not to worry: Just go ahead and use the bike you have, no matter how clunky. In her first triathlon, 10 weeks after she joined the club, Freedland rode 40 kilometers on a 10th birthday present from her grandmother.

She has a new bike now and has finished four triathlons, becoming a committed member of the sport’s 35-39 age group, once its largest demographic.