title: Generate a random birthday given an age
body: <p>I'm looking for a good algorithm, preferably using the CLR's <code>DateTime</code> type and other built-in stuff, versus just using on-the-metal maths, for guessing a birthday date given an age. </p>
  
  <p>The date should be realistic looking, and not just always 1st of January. I need them for testing an app where a number containing the birthday digits is validated with a Luhn check-digit.</p>
  
  <p>What I have so far is to subtract the age from now, giving me a minimum date. Then I can randomize years, months, and days to add. Yet that latter process is rough in terms of getting the wrong random month or day could change the eventual age for the date. </p>