Tapping an American Express card is, more and more, a young person's game. For the first time, millennials and Gen Zers made up the biggest slice of US consumer spending among all Amex consumer cardholders last quarter, CFO Christophe Le Caillec said on the company's fourth-quarter earnings call. They're also the fastest-growing cohort, he said, as Amex works to target younger consumers . To date, the average new consumer customer for the Platinum and Gold cards is 33 and 29, respectively. (The