Boomers could be in for a shock. Although they're the age group closest to retirement, according to a study of 401(k) participants by by Natixis Global Asset Management, 33 percent of them have less than $50,000 put away for the big day, even though they say they'll need a 401(k) balance of $805,000 to cover living expenses in retirement. 

On average, boomers have put away $258,898, only about a third of what they'll need, while millennials — who appear to be underestimating their retirement financial needs because of their age, believing the magic number to be $822,000 — have already done better than that. They've got an average of $91,215 in their retirement plans and years of saving ahead. 

GenX (ages 34-47) isn't doing a whole lot better than boomers, although they seem to have more realistic expectations of how much they'll need than millennials do. Their goal is just over $1 million, while they've saved an average of $203,801. Of course, they have more time than boomers do, but their target is higher.