What’s the key ingredient to a happy retirement? Good health, according to 81% of retirees polled by Merrill Lynch and Age Wave in a new survey. Money helps too. Fifty-eight percent of the respondents cited financial security as the single most important factor. Thirty-six percent said family and friends were the most vital need. And 20% said having a purpose is the key. Health weighs heavily on the minds of the respondents.