I'm not sure what to say here as Statutiory Rape Laws are different all over the world, and different according to circumstance. As far as I can see, there are a lot of factors to consider..
Does the girl look older than 15?
Did the guy deliberately persue her knowing that she was 15?
Was she pressured into having sex with him?
Is he an immature 20 year old?
Is it a case of the mother just not liking the boyfriend?
I'm not trying to say it's anyone's fault, just trying to list possibilities as to why the police are not interested.
I found this link
http://news.bbc.co.uk/1/hi/special_report/1999 /02/99/e-cyclopedia/437789.stm
Hope that it may be of some help.