An under-18 can hold their own Single Trip policy, or travel independently from the other travellers on an Annual Multi-Trip policy.
However, the policy must be purchased by an adult who has responsibility for the child, and the child must also be travelling with a responsible adult at all times (such as a relative, guardian, or school teacher).
As is expected across all Annual Multi-Trip policies, their journey must not exceed the maximum number of days permitted per trip under the level of cover purchased (AMT Duration).