Been through something similar recently.
< 18 need to be chaperoned as has been said. So if both minors no.
Rules say that the guest can be the chaperone according to the lounge manager in Auckland, but having had a really embarrassed and upset 17-year-old and his 21-year-old cousin rejected from several lounges recently it is clear that the lounge staff don't know the rules - their view was the member needs to the 18 plus.