More young people are getting Costco memberships, but they're not spending as much as boomers or Gen X

• More young people are getting Costco memberships, CEO Craig Jelinek told CNBC.
• But they're not buying as much, he said, partly because they might not have started families.
• Younger shoppers have a different purchasing power to baby boomers and Gen X, Jelinek said.

More young people are buying Costco memberships, but they're not buying as much as older generations, the retailer's CEO says.