I don't think you're going to find anyone younger than 30 outside of the big cities. Or, if you somehow do, she will be a 5 or below on the looks scale. As others have said, add at least 10 years to their age (and maybe up to 20). But I'll bet the most common adjustment is 12 years. This is because of the 12 year Chinese year cycle. If a woman says she is a dragon, born in 1988, then I will assume she is a dragon born in 1976.
Asian women can pull this off because they age better than other ethnic groups. They also stay thinner due to their diet. Makeup and the dim light of the spa also helps a lot. Plus the fact that, as Westerners, we are just bad at judging their age vs. more familiar ethnic groups. Seeing them out in the sunlight usually adds 5-10 years to their perceived age. How much their breasts sag is another good indicator.