For starters, most millennials aren't signing up for the right type of plan, she writes on Money: "Many employers now offer two flavors of 401(k)s. Every plan offers a traditional 401(k). … But starting about a decade ago, plans were allowed to offer a Roth 401(k) as well.
"Today about 60 percent of plans offer a Roth version, but less than 10 percent of young adults are saving in one."
That's a mistake. Most young people should sign up for the Roth, she says.