well, I think that 1. its because you would get a certain responsibility that they cant trust on someone who might not be mature enough to handle a certain situation 2. not everybody is always friendly towards staff and I feel like especially younger children can be fairly sensitive towards this kind of behaviour (like getting emotional about it) 3.younger children change their minds more about stuff then older people.. for example, a 19-year-old Minecraft player who has been playing for basically his entire life is less likely to drop the game and therefore drop his responsibility 4.With Cubecraft being a more known server they like to keep things running smoothly and therefore it's important that everyone knows their job and in a helpers case how to handle a certain situation this usually comes with time as well..
I am certain that there are some other reasons that I just didn't think of atm but I hope this is sufficient to answer your question.
Happy to help, Joe