At least 6 percent of Americans between the ages of 44 and 70, or 5.3 million people, are working in second careers with non-profits, governments, schools, or other organizations that benefit society, according to a new survey.And half of the people in that age group who aren’t already involved in so-called “encore careers” say they would like to find such employment — a great opportunity for professions in aging.The survey was commissioned by Civic Ventures, a charity in San Francisco that seeks to engage older Americans in civic activities, and paid for by the MetLife Foundation, in New York. It was based on telephone interviews with more than 1,000 people.