Building a big retirement nest egg can be even more of a challenge if you use faulty strategy. And many young investors do just that. Seventy-six percent of Americans ages 15 to 24 — which includes millennials and members of gen Z — admit they know little or nothing about how to invest, according to a new survey by brokerage TD Ameritrade. Nearly half — 47% — believe a savings account is the best way to prepare for retirement.