(24/7 News) 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 $9.4 billion last year. 

Co-founder and ex-Zuckerberg roomy Dustin Moscovitz is worth about $5.2 billion dollars. 

Thirty-year-old Scott Duncan can claim a net worth of $5.5 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 $4.4 billion.