Have you spent any time reading this or the links at the bottom?
http://forum.sex141.com/eforum/viewthread.php?tid=51284
You have a lot of questions. I think some could be answered by doing some independent research.
But for this question, there are literally hundreds of walkup girls in HK at any time and while there are some prolific punters, I think unless they work for the census bureau they won't know. You can find all age ranges from 20 - late 40s. I would guess most are in their 30s. Certain buildings are known for certain demographics, but the best thing you can do is read reports, post reports, go to the bldgs and see if the girl looks good to you. Who gives a fuck if she is 26 or 36 as long as you think she is attractive and you have a good time.