A 19-year old equestrian enthusiast has been named as the world’s youngest billionaire.

Alexandra Andresen made her debut this week on the Forbes Rich List, an annual estimation of the world’s wealthiest based on the magazine’s investigation of their assets and debt.

Microsoft head Bill Gates kept his number one spot for the third year in a row, followed by Zara founder Amancio Ortega and investment guru Warren Buffett.

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Unlike this year’s top three, who earned their fortune in the fields of tech, business and fashion, Ms Andresen worked far less for her estimated US$1.6 billion.

The beautiful, horse-obsessed teen made her way onto the list because of an enormous inheritance previously unknown to the world.

The Andresens and Ferd

A picture of Ms Andresen jet-skiing in Greece hints at her personal fortune. Photo: Instagram

In 2007, Ms Andresen and her older sister Katharina were each given a 42.4 per cent stake in their father’s billion-dollar investment firm, Ferd.

Their father Johan F Andresen is still alive, but for an unknown reason handed 80 per cent of Ferd’s shares over to his daughters before their 18th birthdays.

Their dad retains a 15 per cent share in the company, but has a 70 per cent vote in its operations.

Norwegian law meant the gargantuan inheritance was kept secret until the girls turned 17, as it is illegal to publish the financial status of minors in that country.

But in 2014, when the girls came of age, the money became public knowledge and the girls found themselves in the public eye.

In an interview with a Norwegian TV station, Ms Andresen said she received over 500 friend requests on Facebook when the news got out.

A cigarette empire

Ferd has not always been an investment company.

The Andresen’s mammoth wealth can be traced back to a Norwegian cigarette empire that began in 1778 when a relative bought a tobacco factory.

In 1849 the family bought JL Tiedemanns Tobaksfabrik, before eventually merging with Skandinavisk Tobakskompagni to become the biggest cigarette producer in Norway.

When Mr Andresen sold the cigarette company for $500 million in 2005, reportedly for ethical reasons, he invested the funds in private investment company Ferd.

Athlete and animal lover

Despite her extreme wealth and new-found fame, Ms Andresen’s Instagram account tells the story of a quiet, focussed and socially-conscious young woman.

Ms Andresen is aiming high – towards the Olympic Games. Photo: Instagram

The vast majority of her posts are pictures and videos of Ms Andresen’s horses, as well as the odd meme about being vegan.

Ms Andresen is clearly an animal lover, posting messages urging her followers not to eat animals or to hurt them.

“Everyone just wants to be liked and accepted,” reads another post. “Except for vegans – vegans don’t give a s***.”

She clearly has a sense of humour, having posted an image of herself and what looks to be her boyfriend pulling funny faces with the caption “Happy Valentines Day”.

While her sister Katharina studies sociology at Amsterdam University College, the younger Ms Andresen aims to represent Norway at the next Olympic Games in four years’ time.

In an Instagram post of her working out in the gym, Ms Andrersen wrote: “Working towards Tokyo 2020.”

Eventually though, it seems Ms Andresen plans to hop off the horse and become involved with her father’s company.

“I feel a great responsibility,” she wrote in a post on Ferd’s website. “I want to help develop Ferd and make it even better.”