The registration process for those running under the age of 18 must be undertaken by a parent or guardian. This person does not necessarily need to participate with them, however the under aged runner does need to be chaperoned on course by an over 18 runner.
When registering the under 18 runner, at the start of the registration you'll be asked who you are registering for. Select the option 'someone else, below 18' and proceed from there. Now you'll be able to select an age under the age of 18, whereas this isn't possible if the under 18 participant tries to register themselves.
See attached image for reference.