Victims of child sexual abuse are not handled judicially. However, if the body of elders believes that congregation action may be warranted in the case of a mature minor who was a willing participant in wrongdoing, two elders should call the Service Department before proceeding.
The issue here seems to be the difference in the 'age of majority' (ie when a person stops being a 'minor') and the 'age of consent', which are not the same.
It appears that generally the age for consent is lower than the age for majority.
For example I understand that in New Zealand the 'age of consent' is 16, but the legal term 'age of majority' is 20.
Hence the above quote to cover the situation with 17, 18 and 19-year-old 'mature minors' in New Zealand.
https://en.wikipedia.org/wiki/Age_of_majority
https://en.wikipedia.org/wiki/Age_of_consent#By_country_or_region