How can you avoid answering the age question not in interviews but when total strangers ask i.e. how old are you?

If you don't want to be terribly rude, try answering with a smile and a question: "Now, why would you need to know that?" or "I don't think that's really your concern, is it?" Or register astonishment and just ask: "I beg your pardon?" If you consider the question itself to be egregiously rude, perhaps you don't need to worry so much about answering politely. A traditional answer to that particular question is "As old as my tongue and a little older than my teeth."