Costco added roughly 2.5 million Millennial and Gen Z household memberships last year.Robert Nickelsberg/Getty ImagesCostco is the latest warehouse club to tout its growing popularity with millennial and Gen Z shoppers.The company said half of new sign-ups were under 40, helping to lower the club's average age.Sam's Club has reported similar trends as younger shoppers look to stretch their budgets.One of the hottest clubs for millennials and Gen Z continues to be the wholesale club.Costco is the latest warehouse brand to tout its growing popularity with younger shoppers, extending a trend that has become increasingly clear over the past few months.CFO Gary Millerchip said that about half of Costco's new sign-ups this past year were under 40 — a figure that works out to some 2.5 million millennial and Gen Z households paying for memberships."This percentage has been growing since COVID-19 and has lowered the average age of our members over the last few years," Millerchip said on Costco's fourth-quarter earnings call on Thursday.In a survey last year, roughly one in six shoppers between 18 and 35 told GoBankingRates they shop at Costco, especially as they continue to reach "adulting" milestones."I admit that I did feel a little older when I became a Costco member," said Dominic Wright, a Gen Z financial planner who told the outlet he's a fan of the company's eyeglasses, contact lenses, and prescriptions.According to Placer.ai Head of Analytical Research R.J.