Baby 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 Natixis Global Asset Management, 33% of them have put away less than $50,000 for the big day, even though they said they'll need a 401(k) balance of $805,000 to cover living expenses in retirement. 

On average, boomers have saved $258,898, only about a third of what they'll need. 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. 

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