Yes there is a lot of prostitution, the economy is based on it ever since the US military started coming here, but underage prostitution is illegal here

I have no doubt there is child prostitution here if you look hard enough as it is in every city in the world. I know that wont sell many newspapers in the west but thats the way it is.

According to the US-based research institute “Protection Project”, estimates of the number of children involved in prostitution living in Thailand ranges from 12,000 to the hundreds of thousands (ECPAT International). The government, university researchers, and NGOs estimated that there are as many as 30,000 to 40,000 prostitutes under 18 years of age, not including foreign migrants (US Department of State, 2005b). Thailand’s Health System Research Institute estimates that children in prostitution make up 40% of prostitutes in Thailand.