We do moderate this rule,
However, exceptions can be made, with your example for instance.
If this person is mature and behaves themselves then yes they are welcome to Last Moon. It would be especially nice if an older player, such as an older and mature sibling or friend, could supervise their play/play with them, even a member of staff would be happy to be with them/befriend them on the condition they follow the rules. However, if this under 13 does not follow the rules then they will be permanently banned.
The rule is there because the majority of 13 year olds cannot behave in a manner that a mature person could in a crowd of people, such as them mucking around in default as we see now, but we cannot obtain substantial proof of them being under 13 unless they say so themselves. Some children under 13 would also find it difficult to respond appropriately to circumstances in-game, such as an item theft without getting upset, and a lot of them skim past the rules we have here on the forum or totally ignore them.
If this person in your example is, like Dutch said, is really young then there is no way they could fit into Last Moon as an appropriate adult and therefore would be banned.