Even if you guess the marital status "right", you will still offend many. Why should women be defined according to marital status, especially when men are not?
I also never said we should or shouldn't use the terms. I don't have an opinion on that and it's not a debate or argument I'm going to waste my time on. I was simply explaining how they are conventionally used. This kind of thing was taught in schools as recently as 20 years ago as part of explanations on how to write and format letters.
Edit:
It seems using it until the age of 12 is a convention in the US. Using it until 18 is the convention in the UK. Another argument for why software shouldn't make decisions using terms that can mean different things to different people :)