"As he was under 18 the maximum sentence he could have faced for this offence was 5 years. And if his story that she had been buying alcohol stood up he would have been able to argue that he reasonably believed she was over 16 which would mean that he was not guilty."
It doesn't work like that in reality. As a 17 year old up for what the police perceive to be a minor offence (possession of personal use quantities of drugs, or whatever), they will have you in and say 'Accept this caution or we'll ruin your life' [i.e. with criminal record, prosecution, etc.]. So you do.
Whether he could have proved his innocence isn't really the issue - in that scenario, the rational choice is to accept the caution.
The ramification of that, I suppose, are that if he has sex with ANOTHER underage girl, then you would be able to say from the previous caution that it's a pattern. But if he was up for something entirely unconnected eight years later, you can't draw too many conclusions.