when I visited Macau back in the 80's, most of the Thais were available at the massage parlours;
sitting behind those one way mirrors;
and learnt that although many are between 25-30, they are considered too old for the local Thai market. At that time, 15-18 years were most sought for.
As for the present market, most would be between 20-25.
(Seems the older generation likes those hairless girls, although I don't know why; there were some scandals regarding those DOM (dirty old man) caught; in most cases, because of the legal system, many get away simply by paying a small amount of c@$h.)
Don't bel be very surprised to see PRC girls between the age of 35-50 hanging around Chinatown in Bangkok still walking the streets