Unattractive old men are easier for these girls to control. Old men are less demanding, predictable, and generous with tips. It's just a job at the end of the day for them.
Young or hot guys = worse service. Why? Because they're often ego-driven, cocky, terrible in bed, or just plain annoying. If the guy's too attractive, the girl might even get nervous. Young people are stingy too with tips, always trying get their money's worth, demanding 2nd or 3rd round. You get the idea
So if you're an old bald farang, you might actually get the best service.