I had a long discussion with a college friend about this a few years back, when I discovered that he was almost ten years older than I, and not about my age, as I had assumed.
Generally, age lines and facial skin changes on Asian faces occur in slightly different places and/or to a lesser degree than on Caucasian faces. Therefore, the visual cues that Caucasians have been socialized to look for (subconsciously) to determine age are misleading on an Asian face.
At least, that's the theory he and I came up with.
But since I am equally terrible at guessing people's ages of all ethnicities . . .