Daniela Barnea, who is 73, typically swims for up to an hour and a half, seven days a week. At her age, that kind of workout, during which she covers nearly two miles, is noteworthy.

Even more so is the fact that Ms. Barnea, who lives in Palo Alto, Calif., is a record-breaking swimmer and senior athlete who competes in sanctioned races for her age group in events around the world.

At the 2017 United States Masters Swimming Spring Nationals, in Riverside, Calif., she won three gold medals in the women’s 70-to-74-year-old age group. These included the 100-yard and 200-yard breaststroke finals and the 200-yard individual medley finals.

Hundreds of thousands of senior athletes like Ms. Barnea compete regularly in athletic events throughout the world, including the National Senior Games Association, United States Masters Swimming and USA Track & Field Masters programs. Races are divided into five-year age increments starting at age 35 and ending at 100-to-104. Generally, the athletes range in age from 50 to 100.