Bob lives in the Northwest region of the United States. He retired at age 62 after 30 years of work in a management position for an aerospace company. He and his wife of 40 years carefully saved money so that they could be very active in their retirement. They now spend their winters as "snowbirds," traveling in their mobile home to the "sun belt." Now at age 69, they have spent 7 years in the same community in Arizona where they are well-known and have made many friends. In the summer, they usually take one extended trip to the mountains. They enjoy good health and believe that keeping active is the key to their zest for life. The theory that best describes this scenario is .