The situation in Chiang Mai is more complicated...
I was at Free Guy a few nights ago -- a lovely show badly marred by some very loud, drunken Australians -- and asked my friend there whether the bar hosts were now one hundred percent Shan. He nodded yes.
You will also find many Shan at Circle Pub and at My Way. The Shan (Tai-Yai) are typically "illegal immigrants" and do NOT have a Thai ID card. They will have another card -- a work permit (permission to work within a specified place, and may God forgive you if you leave that place).
I am not an expert on this system of identification, but I have noticed that a number of boys "magically" become 18 once they get that card. My guess is that Thai law forbids giving such cards to anyone under the age of 18, but when sufficient tea money is present, the card is issued and the boy automagically turns 18.
That would leave a quandary for those into the younger lads: your Romeo for the evening may be sixteen in fact, but he carries official, non-fake Thai ID stating that he is eighteen or older. It doesn't sound much like a situation I would like to be involved in! But what happens if the only ID you can check is inaccurate?? You could always just ask the guy, but I wouldn't count on the truth from someone looking for an off.