Luxury stores had been feeling tremors for several years as shoppers began to search for their Saint Laurent and Balenciaga goods online, but 2017 was a wake-up call. There’s good reason to pursue young shoppers. They are driving the luxury market, which bounced back last year after shrinking in 2016. A study by Bain & Co. said 85% of the growth in the luxury market last year came from consumers age 35 and younger.