I certainly don't condone adults having sex with any girl under 18. I think that's a good general rule to go by. However I've been here 11 years and am yet to see any clear information on what the actual age is.
I've heard 15, 16 and 18.
People are often also bound by the laws of there home country regardless. For example the Australian passport ships with a leaflet explaining you can't have sex with anyone under 16, regardless of the countries laws.
Although it's common sense for most people, I do think Thailand could make it much clearer.