It's illegal in China, but enforcement varies as people have said here. It can get a little crazy in smaller cities. Its very common in these cities for the hotel to have a relationship with a pimp, and for someone like the concierge or a bellboy approach you after you arrive with a list of their girls with photos. But be careful as a foreigner there. I've gotten in trouble twice.
Once where i didnt like the girls offered and brought my own in later and got reported in to the cops, which actually caused the girl more trouble than it did me, but it was a mess.
The other time this bellman knocked on my door with a string of girls who appeared very underage to me (like 12). I went to the cops over it myself, but it turned out that the pimp was the brother of the chief of police and i ended up in jail overnight. I should have known better than to expect the cops would be honest.
Its ok in the big cities though and I'd single out Shenzhen and Shanghai as excellent experience with vast quality improvements over NYC.