When is it appropriate to use 剩女?
In my opinion, it is usually offensive to call someone 剩女 in the face, even when joking. Of course there might be exceptions, for example, a senior family member may possibly use the word 剩女 to urge a child to prioritize marriage, but even in this case the person referred to may feel offended. My advice is to use other expressions that are more euphemistic.