I worked in Childcare for 30yrs... ten of which were with a group of teenage girls.. which could number up to 42 in total. I was the youngest and the most attractive of the males in the establishment (sorry, not bragging but it was a fact) and many of the girls had a crush on me at one time or another. Although there were girls who I found attractive physically, I would never have abused my position, as a parent substitute, (In Loco Parentis) to indulge any desires I might have... simply because it was against the law and not 'the done thing' morally in any case. However, as I said, I found some of the girls attractive and therein lies the problem... for SOME men. There are some men who are sexual opportunists and will take chances just to gratify their lusts with any woman who is the least bit willing and including for some... women or girls of any age.... Each person who works with underage children in the UK has to undergo Police and background checks... but sadly these checks don't prove a persons character or reveal their inner desires so the problem of 'Adult, child' relationships will rear its head occasionally.
On a personal level, I see no problem with an 18 yr old and a 24 yr old having a relationship but if there are legal reasons why they should not... THEN THEY SHOULD NOT!!!!!
Best wishes, Mike.