You can just find a lot of trails. Then, there's BMX riding, which most people think is just for kids. BMX is the sprint of off-road bike racing. Races are rarely more than a minute long. BMX is the cousin of motocross dirt bike racing, but without the noise.
Racers negotiate a course pocked with jumps and turns.
They need a myriad of bike skills, not only to win, but just to stay on the bike. Both my sons play football and soccer, and there are more injuries in one season in those sports than in BMX for the year. Age groups are grouped by year starting at about 4 or 5 up to 18. After 18, age groups are put in 2 to 5 year groupings. The professional classification starts at 18.