I can’t click around on my iPhone for 10 minutes without landing on an article about generations and advertising. It’s an obsession with marketers these days: What makes baby boomers, gen-Xers, millennials, and post-millennials—plus the babies born yesterday—tick? What’s their buying power, and their favorite way to shop? How do they engage with a brand? And which channels reach them? Are gen-Xers connecting on Facebook over kids’ prom photos? Of course. Are millennials streaming Riverdale on The....