The day before Thanksgiving is one of the busiest for air traffic, so don't even think about stretching out in coach for a nap on your flight. Who may be sharing the armrests with you? Older Americans - they travel the most and, according to the new book Best Customers: Demographics of Consumer Demand (New Strategist Publications), they spend the most on airfares. Householders aged 45 to 64 spend 32 percent to 36 percent more on airline tickets than average, based on an analysis of the Bureau of Labor Statistics' 1997 Consumer Expenditure Survey. Married couples with adult children at home shell out 71 percent more than average on airfares.