Costco has a new growing customer base: members under the age of 40.  CFO Gary Millerchip told investors during its earnings call at the end of September that the membership warehouse is gaining an influx of younger shoppers. Almost half of its new member sign-ups during the three-month period ending on Aug. 31 were under the age of 40, Millerchip said on the call. The company also reported a 92.3% renewal rate across North America. The rate is down slightly from the prior quarter due to more people joining ...