You are right with the +10 years, and this applies to most Asian girls.
If they look after themselves, eat right, exercise, laser out skin blemishes, use botox and expensive anti-wrinkle creams like La Mer, La Prairie et al then you can add 15-20 years to the presumed age.
I'm smashing raw a 50+ Korean sugar mama that looks and feels like a 30yr old (a good thing she owns her own cosmetics and beauty treatment biz)