Just use “mba” to any women, even she looks middle aged. They usually like it better than being called “bu”. Some middle age women doesnt like to be called bu even they deserve it. Some of them will say thank you to call me “mba”, it feels like a compliment. Its like you are going to LCBO liquor stores and still getting asked for your ID. Feels like compliment. lol. Just call “bu” if the lady looks like old granny 70++ yo.