Once upon a time, the role of CEO was near-enough exclusively reserved for middle-aged men. 

But in the new world of work, the situation is different. Exciting new tech startups can spring up and propel their young founders into the spotlight overnight. It doesn't always take years of climbing the career ladder to earn success anymore. 

Here are five millennials leading the new generation of business leaders…

Evan Spiegel, CEO Snap Inc

The son of two Californian lawyers, Evan Spiegel quit Stanford University just a few credits short of completing his degree. Why? A more interesting project was taking up his time.

In 2011, the then 22-year-old Spiegel, along with fellow students Reggie Brown and Robert Murphy, started work on an image messaging app. Then called Picaboo, the app was unique because it deleted its messages after they were sent. The idea took off. By 2012, Spiegel was rejecting a $3bn buyout bid from Mark Zuckerberg for the app – now called Snapchat. 

Spiegel's expertise lies in design (it was his subject of study at Stanford), and while he doesn't know much code and lacks formal management training, he is said to be an imposing leader. 

In his own words, he said at an annual Code Conference: "I’m very decisive but I change my mind a lot, which is sort of a unique combination. But it really means that we care about making decisions really, really quickly, but we care about the flexibility to change our mind, and we want to make sure that is the key component of our business." 

David Karp, CEO Tumblr

When Manhattan-born David Karp upped sticks and moved to Tokyo at age 17, he originally wanted to build robots. But soon, his restless creative mind settled on numerous other projects. 

Among them was a microblogging platform called Tumblr. Returning home to New York in 2007, David set about dedicating his efforts to the platform full-time. Within the first fortnight of Tumblr's launch, it gained 75,000 new users. 

At first, David was all-too aware of his youth and inexperience. He was reluctant to reveal just how young he was, even going to lengths like feigning a deeper voice over the phone and dodging face-to-face meetings with investors. 

Having turned 30 last summer, it's safe to say he hasn't let youth or inexperience interfere with business. As of 1st January 2017, there are more than 329 million active Tumblr users. Karp's estimated worth meanwhile is around the $200 million mark.

Anthony Casalena, CEO Squarespace

Back in 2003, University of Maryland student Anthony Casalena decided to make his own website builder. Basically, he wasn't happy with the current range of platforms on offer.

Seemingly abiding by the rites of passage for all millennial tech CEOs, he eventually founded his company Squarespace in his dorm room. 

During the company's early years, he acted as the sole engineer, designer and support representative. But after three years of the lone wolf approach, Casalena was struggling. So he hired Dane Atkinson to serve as Squarespace's Chief Executive. That gave him the opportunity to continue his day-to-day operations while taking some time away from the spotlight.

That cooling-off period between 2007 and 2011 turned out to be a time of huge, rapid expansion for Squarespace. Today, he estimates the platform gets 1,000 new customers each day. 

Sean Kelly, CEO HUMAN Healthy Vending

When Biomedical Engineering student Sean Kelly was in the gym one afternoon, he noticed someone head to the vending machine and buy a can of Coke. Upon closer inspection, he noticed it was the only drink on offer.

The irony of a gym only offering only sugar-filled beverages wasn't lost on Kelly, so alongside co-founder Andrew Mackenson, he set about changing things. He set up HUMAN Healthy Vending – machines packed with nutritious treats instead of sugary snacks. 

With the ambition of becoming America's leading anti-obesity company, by 2014 HUMAN was ranked the world's fastest-growing vending franchise by Entrepreneur Magazine. Today, there are HUMAN machines scattered across the US, Canada and Puerto Rico. 

Kelly has said of his success: "Do not be scared to surround yourself with people smarter and more experienced than you. You need people not to push you, but to pull you. Get the mentors, get the leaders, and do not be afraid to copy success."

Megan Grassell, CEO Yellowberry 

Megan was 17 years old when she took her little sister shopping for her first bra. She noticed what was on offer wasn't age appropriate – everything came with padding and uncomfortable underwire. So she decided to do something about it.

She set up Yellowberry, a retailer dedicated entirely to bras for young girls – complete with fun colours and comfortable materials. 

It wasn't easy at first – Megan had to learn everything from scratch, from how to sew to how to source fabrics. But after moving from prototype from production (thanks to Kickstarter), she eventually started manufacturing products in her hometown of Jackson Hole, Wyoming. 

Speaking to Fortune, she said: “I have a vision for the company. I know how I want it to grow, and I can’t wait to see how it becomes established.” Now 19, Megan deferred her first year of college to continue her role of CEO. She would have probably cracked open the Champagne if she was legal drinking age!