you can certainly ask...don't forget, you are the customer! They may not provide a lineup, they may not have many girls working or available at the time, could just be their policy not to provide lineup. It is then also your option whether you decide to remain or go elsewhere.
As for your age preference, I have found that there are a lot more providers in these places who are over 30 than under. The good news is asian women look a lot younger than they really are, so you could well end up with a 40 year old and think she's only 30.

I agree with olepapapa, I know a 40 year old Filipino girl who looks 25! Plus the older ones have more expeirience...

Have my vote on this one. When you,the customer walk in a store, you choose.