Maybe Argentina? The age of consent is 13 and prostitution is legal. Even with this low age, they have a serious problem with child prostituion - and human trafficing. But then, where are prostitutes happy and not exploited? I really hope this question is hypothetical and not something you're into, because using other people like this is selfish and sick.
In the Netherlands the age of consent is 14
They need to be 16
That would be Kentucky.