They welcome younger volunteers (who are old enough to read and follow directions obviously) but what they assign you to do will depend on your age. Because of "Safe Sport" (a set of rules they have, to keep minors safe) they won't put minors in a situation where they might be by themselves, or alone with non-family adults. But I've seen 11 or 12 year olds jump judging in pairs or groups, whose parents are nearby, like riding in the show or doing other volunteer jobs.
If you are a minor and go by yourself, like if your parents drop you off and pick you up later, the show organizers will put you somewhere very public, like opening the gate to let horses in and out of the warmup arena, or being part of a crew that puts knocked-down poles back up in the show-jumping. And they'll want you to go on bathroom or food breaks in groups.
