Right.
As pp say, it is not a comparable situation, but here goes.
If the 18 year old and the 14 year old were living here permanently, I would take the 18 year old aside and advise them they were breaking the law and that they could be prosecuted. I would report if I thought the 14 year old was being coerced/not consenting. If not, I am not sure whether I would report it.
Eta: if the 18 year old was in a position of trust, as set out in your later post, I would report.
If the man was in his 20s I would report with no hesitation.
If they were here on a three week holiday, I would keep my nose out unless I thought the 14 year old was being coerced/not consenting. Although if they were staying under my roof I would not allow it.