There are over 15 regions or associations that they break up the united states into. Each region only runs against they own competitors until nationals, where all regions come to compete and win a national title. 21 people from each age group can go to nationals from each association, but only 5 people from each age group can go for track nationals. You can start competing when youre 3 and you stop running Junior Olympics when youre 18. At track nationals you can have over 100 people running against, but it is broken up into heats. At cross country nationals you can be racing against over 300 people IN THE SAME HEAT! The cross country courses can range anywhere from 3,000m-5,000m at nationals, depending on your age group.