They may be the most closely tracked generation in human history. But it's remarkable how much we've gotten wrong about millennials.

Marketing strategies devised for millennials by older generations have been based on bad intuition, poorly curated data, and unquestioned, incomplete assumptions. That leads to bad guesses. When you're designing banking and payment systems for your next generation of customers, bad guesses can be expensive.

For instance, it is accurate to call millennials (a.k.a. Gen Y's) the original "digital natives." They were the first generation to grow up online, with a comfort with networks that spawned a radically distributed approach to commerce. Peer-to-peer sharing, a threat to existing models, was an opportunity for new entrepreneurs.

By letting millennials reframe peer-to-peer payments, apps like Venmo seemed poised to disrupt one of the primary roles of banks. Rather than fight shifting tides, the industry sought to be the ones surfing the P2P payment wave. Zelle, the supposed Venmo-killer, looked like the right move for banks to retain millennials,

But is a millennial sea change in payments reallyunder way? Does that track with the data? Or is it another bad guess?

Certainly, some Gen Y's have gone all in with phone-based payments. Most, however, are just not that into it. 68 percent of young adults do not use any P2P payment platform at all.And out of the 82 percent of millennial iPhone users who had heard of Apple Pay in another survey, only 8 percent use it.

The case for nouveau payments is just not all that compelling for some. Others may be waiting for the right sweet spot of form and convenience. For now, even among millennials, cash remains king and the traditional payment forms of cards and checks still carry the lion's share of their transactions.

In a 2016 Cardtronics survey, nearly half of the millennials queried said they're more likely to pay with cash now than they did a few years ago, even though most said they were using a greater variety of new payment methods and P2P apps. Millennial usage of low-tech currency increased at a greater rate than any other age cohort, too.In another survey, almost two thirds of millennial respondents said cash was their preferred medium for P2P payments.  And the second-most preferred method was a dead heat between PayPal and checks, of all things.  Venmo came in a distant fourth.

Another sweeping generalization about Generation Y is that it's a demographic monolith of bankless, networked nomads. But, in fact, millennials behave like consumers from other generations when they come of age.

Let's step back and consider Generation Y demographics: Born between 1981 and 1997, millennials are now 20-36 years old. That means department managers, VPs of sales and CEOs of successful startups—probably juggling kids, mortgages and retirement savings—are getting lumped in with college juniors. But older millennials actually have a lot more in common with their Gen X or even Baby Boomer colleagues than they do with Gen Y's in their early/mid-20s, who have consumption and media habits that mirror Post-Millennial Generation Z.

Young Gen Y's may have breezed through college with nothing more than a phone and a couple of payment apps. As they take on life responsibilities, however, like full-time employment, mortgages, investments and savings, Millennials will need their banks just as much as earlier generations. In the developed world, banks are still the very medium through which business gets done and adult lives are lived.

Banks cannot sit on their laurels as they once might have, of course. Digital natives have expectations shaped by instant fulfillment, complete mobility, and the unrestricted choices made in global social networks. To serve these customers in a way that is native and intuitive to them, banks will need to listen and innovate. 

Yet banks should also keep in mind that as the young millennial's life responsibilities evolve, he or she will take on the kind of personal and professional lives in which banks are uniquely suited to play an integral role.