NEW YORK—Wiping a tear from the corner of his eye Thursday as he reviewed Nielsen ratings and various market performance indicators, advertising executive Mark Gosserman wistfully recalled how his clients’ target demographic of middle-class consumers with disposable income had grown up right before his very eyes. “It seems like just yesterday they were eager 18- to 34-year-olds with robust entertainment and apparel consumption habits who represented over $200 billion per year in direct purchasing power,” the emotional executive said, growing nostalgic as he watched a brand-driven television commercial featuring a celebrity endorser and a contemporary pop song that his target audience used to love. “Now look at them—all grown up and watching 143 hours of television per month. What I wouldn’t give to go back in time to when I was still aggressively marketing mobile devices and energy drinks to them.” Gosserman added that he looked forward to seeing the market segment mature until reaching 50-plus years old, after which his firm would ignore them altogether.