Do you need a coach, or someone over 18 to compete?
You need a parent/guardian to compete at a competition, as it is required to sign up at the competition. If you just show up there with no one considered a “parent/guardian” you can not compete.
Vex requires at least one adult to be responsible for a team, but you really don’t need a very involved coach, just someone to show up at competitions.
they don’t even need to show up. Technically you could print of the forms, have a parent sign it at home and turn it in at the competition.
At most events we have been to only adults are allowed to register at the event. That ensures that one adult is there, or at least dropped them off.