I think men who use clearly under-age girls should be prosecuted. Otherwise, the law of supply and demands operates. My daughter used to work in Drug/Alcohol abuse services and she says that many of her clients used prostitution to fund their habit - but of course it can happen the other way round, where a girl forced into the sex 'trade' turns to drugs to soften the reality of her life.
I wonder if it would be possible to set up brothels away from residential streets (say, on industrial estates?) where women could have some protection and medical/social workers could visit?
Incidentally, prostitution is defined as a trade for income tax purposes (burglary is not, as there is no implied contract between the two parties concerned). It is quite hard to check income, however!