It's now time for Part II of the most wealthy Americans under 40. To see Part I and rankings 15-8, please check out Part I - The Top 15 Most Wealthy Americans under 40.

7. Brian Chesky and Joe Gebbia

Both at 33 years of age, Brian and Joe are the founders of AirBnB, the bed and breakfast service where homeowners open their homes up to guests. Brian and Joe have found their fortune quick, both worth 1.9 billion after only six years in the business. Over 30 million people have used AirBnB since it started over six years ago, and 20 million of those came in 2014 alone.

6. Scott Duncan

Scott is the youngest of four heirs of the Duncan family. His father founded an energy-pipeline empire, owning over 50,000 miles of natural gas, oil and petrochemical pipelines. Therefore, Scott is a billionaire only by inheritance, which makes him atypical on this list since most other people are self-made billionaires. Scott is 32 years old, lives in Houston, and is worth a whopping 5.6 billion.

5. Nathan Blecharczyk

Nathan is the third cofounder of AirBnB. He finds himself higher on this list because he is the youngest of the three. Nathan is 31 years old and worth 1.9 billion.

4. Tatiana Casiraghi

Next on the list is our first of two females. Although only two of the top 15 wealthiest Americans under 40 are women, both are in the top 5. Tatiana is an heir of her grandfather Julio Mario Santo Domingo who owned Columbian brewery Bavaria, and therefore, not a self-made billionaire. She is, however, worth 2.2 billion and is only 31 years old.

3. Elizabeth Holmes

Worth 4.5 billions, Elizabeth is the youngest self-made billionaire woman. She dropped out of Stanford to start a blood testing company called Theranos. If you visit one of the over 8,100 Walgreens throughout the country, look for Theranos Wellness Centers inside their pharmacies.

2. Dustin Moskovitz

Zuckerberg's Harvard roommate and third-ever employee of Facebook, finds himself at the second spot on this list. If you remember, Dustin actually left Facebook in 2008 to start his own software company Asana. He's 30 years old and is worth 8.2 billion, which puts him at the 59th richest person in America. Dustin has been known to catch some attention for biking to work and flying commercial, as well as donating millions to fight malaria and other diseases.

1. Mark Zuckerberg

We all know the name of Zuckerberg and how Facebook made him not only famous, but one of the richest people in the world - 12th in fact. Mark is 30 years old and worth over 34 billion. The most astonishing thing about this is that he is far and above every other American billionaire below 40.