On paper the ages look awful but a/ lots of young girls, rightly or wrongly, do have sex at such an early age, especially if they are in care; and
b/ he might have been 18 in age but mentally, he might have been much younger. For those who are asking who was in loco parentis, if she'd been in care, the authorities would have turned a blind eye.
One of my stepsons got involved with a 14 year old when he was 18. Although her parents were around, the Social Services were involved and they quite happily sanctioned the girl living with her boyfriend. We were horrified but as he lived 200 miles away and his mother was ok with it, we could do nothing about it. Eventually she got pregnant at 15 and we were terrified for what would happen to him. Social Services, once again, did nothing because she would be 16 when our grandchild was born.
Now, I do not condone my stepson but there was absolutely no doubt who was in charge in that relationship to the point where she was abusive. Emotionally, he just didn't have the wherewithal to cope with her and was completely terrified of her temper. He also didn't have an adult in charge who said that underage sex was wrong but also facilitated it.
He made the best of it until he caught her putting her birth control pills in the bin when the child was a year old and he had been adamant he didn't want another baby until they were more settled. Fourteen years later he does a 400 mile trip around the country to get access once a month because she moved away and has paid maintenance dutifully. Meanwhile she has had several more children with several different fathers.