There have been many casualties of the global economic slump. But the economy hasn't stopped another war: the one against aging. In fact, in some ways, economic troubles have helped Nu Skin Enterprises (NUS), the direct seller of anti-aging skin care and nutrition supplements. Its global network of active distributors, those who buy its products and sell them Tupperware-party-style to their friends, held steady through 2009, at 761,000. But the