Twenty of Forbes top 400 hundred billionaires haven't hit the age of 45 yet.  The publication released its "Forbes 400 List" this week.

At just 29-years-old, Facebook founder Mark Zuckerberg is worth about 19-billion dollars.  That's up from nine-point-four last year.  Co-founder and ex-Zuckerberg roomy Dustin Moscovitz is worth about five-point-two billion dollars. 

Thirty-year-old Scott Duncan can claim a net worth of five-point-five billion as one of four heirs to an energy pipeline fortune. 

Investor Ken Griffin is the oldest of the youngest.  At just 44-years-old, he's worth four-point-four billion.   (Metro Networks Inc)