The Answer:

According to the Statistical Abstract of the United States, in 1997, 45,119,000 people said they participated in bike riding at least six times during the year. Of those, 19,699,000 were children (ages 7-17) and 25,420,000 were adults (18+).

According to the Abstract, bike riding was the fifth most popular recreational activity for Americans, trailing only exercise walking, inline skating, working out with weights and camping.

—The Fact Monster