When is it appropriate to use 剩女? For the average Chinese woman approaching the age of 27-30, getting married is a fairly high priority, and a large motivating factor to this is the cultural pressure of avoiding becoming a 剩女, or leftover woman. 

The word is used often enough in news articles and television when talking about girls who are nearing this tragic fate, but when, if ever, would a person specifically refer to someone directly as a 剩女? It's obviously not a compliment, but in regards to culture in Mainland China, could it ever be used jokingly (你今年都25岁了，快会变成剩女吧)? Or when concerned about a friend's future (你现在不年轻了，你要快点结婚，不然变成剩女)? 

So when, if ever, can someone explicitly refer to someone as a 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.