The answer might depend on your governing organization. According to USEF (and any group who follows USEF's rules) Juniors are anyone under 18 years old, before Dec 1st of the competition year. Once you're 18 you are considered either an amateur or a professional. (juniors can't be pros or ammys). There is a division called Children's Hunter and usually there are 2 age groups: the youngers (13 and under) and olders (14-17). There's also a division called Children's Hunter Pony, and I believe any junior can ride any pony, but I"m not certain (in the Regular Hunter pony divisions only certain ages can ride certain heights). Does that help?