...How come you are still single? Actually it would be a little rude to respond with any question about someone being single. As a rule, an older person who is single is usually divorced, and no one wants to bring that stuff up. Otherwise it is considered a little rude to question one's chosen direction in life.

Hey Davood, I would say "Why have you remained single until now?" or, if you want to use your wording, "Why have you remained single until this age?"

Its not polite to acknowledge a woman's age, hint at it and definitely not ask for it - unless they are on very good terms with you. The possible way to broach 'why is someone still single at a particular age' in conversation would be to act exasperated they are still single. For example if it comes up in conversation you could say words to the effect of "I can not believe someone has not snapped you up" OR "you must be single because your picky, because it can not be because of lack of attention." This may then roll the conversation into the territory your looking for :)

"Why aren't you married yet?" Agree with Eliot. Better don't ask.

"why do you ----" should only be used when asking someone why they do something they've done before. So, it's actually "Why did you remain single till this age?"