Even if you look as good as Andy Lau and Aaron Kwok, they might still call you uncle as long as you look mature. Some of them have their own agenda in calling other people uncle. For example, want to pretend to be young or act cute. Some have simply forgotten that they themselves have grown old and are no longer entitled to use such terms. If you really look as good as Andy Lau and Aaron Kwok, who are in their early 50s and late 40s respectively, then it's possible that a third group of "kum tao kum bin" will surface and purposely call you uncle, especially in front of other customers. They are simply jealous that even though you look about their age, you nevertheless look much better either better than them or their spouses.
You must educate these people by giving them a piece of your mind. Say something like 你叫我 Uncle 我受不起啊!我最大的 nephew 都还不到 18 岁。我没有这么好的福气有像你年纪这么大的 nephew, or if the other party is an auntie, 有像你这么老的 niece.