The term "generation gap" was popularized in the 1960s, when young baby boomers started coming into conflict with their parents' values. But now, marketers are reviving the generation gap to sell stuff to both boomers and millennials.

Going back as far as the '60s, Pepsi used the generation gap to draw teens away from the cola their parents drank.

Twenty years later, when those teens were parents themselves, Pepsi started targeting their children's generation by turning to the King of Pop — Michael Jackson — to brand Pepsi as the choice of a "new generation."

And in 1969, a retailer went so far as to name itself after the generation gap. In a 1991 ad, for example, The Gap used words and clothing styles to make one generation feel superior to another — proclaiming that to be part of a specific generation "is much more than the simple matter of your birthday — it's to be part of an era."

Millennials made an early appearance in a 1996 ad for Nintendo's Game Boy, which showed youngsters clashing with their grandparents' generation — though they do reconcile by the end of the ad.

And in a 2011 Toyota ad, the current gap between millennials and baby boomers is highlighted. A young woman sits alone at her laptop, lamenting her parents' "anti-sociability," as evidenced by their low Facebook friend count. Meanwhile, those parents are seen driving out to the country to mountain bike with friends.

It's easy to tell which generation the Toyota ad is targeting. Instead of sitting inside all day with virtual friends, the boomers are out in the real world being active — and, of course, spending money on new cars.

For now, most major marketers would choose this same side of the gap. While the millennial population is as large as the boomer population — and beginning to emerge into its peak spending years — there's a problem.

Recent economic crises have hit millennials hard, so their spending power hasn't reached full potential.

Still, some marketers come down squarely on the side of millennials. A recent Samsung Galaxy ad, for example, celebrates millennial mannerisms — explaining the predilection for photographing food, using phones for anything except talking, and selfies with "because it's what we do."

Right now, marketers are walking a tightrope directly above the generation gap. 

Somehow, they have to ingratiate themselves with millennials as future prospects, without alienating baby boomers who — for now — continue to dominate spending.