These billionaires weren't born some of the richest men in the country—they started out as everything from gum salesmen to bartenders. But they've worked their way up in the industry, acquiring famous real estate assets and founding companies that continue to blaze new trails. From Warren Buffett to Mark Cuban, see how these five billionaires got their starts.

1. Warren Buffett

Age: 85
Net Worth: $63.3B
Founded Companies: Buffett Partnership Ltd

The Oracle of Omaha is well-known as perhaps the most successful investor in the world, and he got started early. Warren Buffett bought his first stock at age 11. But he didn't become a billionaire overnight, instead he has been working toward it his whole life, from selling chewing gum as a child to his first post-college job as an investment salesman. Buffett has always held outside investments—his first was purchasing a 40-acre farm while still in high school.

2. Sam Zell

Age: 74
Net Worth: $5.1B
Founded Companies: Equity Group Investments, Equity Residential, Equity Office, Equity Lifestyle Properties, Capital Trust

Many people know Sam Zell has played an enormous role in shaping the modern commercial real estate industry, but he's been interested in business since childhood. When he was only 12 years old he would buy bulk copies of Playboy for 50 cents each and resell them for between $1.50 and $3. Zell took his first steps into real estate while at law school—he managed student apartments for a variety of landlords and flipped distressed properties.

3. Donald Trump

Age: 70
Net Worth: $4.5B
Founded Companies: Trump University, Trump Entertainment Resorts, Trump Shuttle, Trump Model Management, Trump Mortgage, Trump Productions

Best-known today as the GOP 2016 Presidential nominee, Donald Trump was born to Fred Trump, one of the largest real estate developers in Queens. His real estate career started early—during college he worked with his father to revitalize a foreclosed apartment complex in Ohio, and with a $500k investment they managed to turn around its occupancy rate from 34% to 100%. Trump's business interests have expanded over the years to many areas outside of real estate.

4. Larry Silverstein

Age: 85
Net Worth: $3.5B
Founded Companies: Silverstein Properties

Like Trump, Larry Silverstein was born in NYC and first started working in real estate with his father. In 1957 he bought his first building with his father in Manhattan and continued to work with him until his death in 1966. Over the years Silverstein continued grabbing up properties in NYC, and in 2001 he made a successful $3.2B bid for the lease to the World Trade Center. Since the terrorist attacks of 9/11, he's been focusing his efforts on the reconstruction of the WTC towers.

5. Mark Cuban

Age: 57
Net Worth: $3B
Founded Companies: MicroSolutions, Broadcast.com, 2929 Entertainment

Mark Cuban entered the business world at age 12—he turned a profit selling garbage bags so he could afford cool basketball shoes. After college he worked as a bartender before finding work as a salesperson for one of the first PC software retailers in Dallas, but after less than a year on the job he was fired for trying to get new business from a client. He went on to found MicroSolutions, which he sold a few years later for $6M, giving him enough cash to fund his next enterprises.