Facebook founder Mark Zuckerberg holds the title, after joining the so-called Three Comma Club at the age of 23 a decade ago.
Kylie, the youngest of the Kardashian/Jenner clan, is estimated to be worth 900 million US dollars ($AU1.213 billion) and counting, mainly thanks to the cosmetics range she launched in 2015.
Her standout success is her Kylie Lip Kit — a matching lipstick and liner costing $AU37 which sold out in seconds at its launch.
Fans desperate to imitate her trademark "perfect pout" who missed out were even paying up to $AU1333 for one on eBay.
Kylie, known for her plump pout, began using fillers aged 16 after a boy told her she was a good kisser for someone with such small lips. She has recently stopped.
She describes her business as "the most authentic thing I've done in my career", adding: "It came from an insecurity and I turned it into something.
"I was insecure about my lips, and lipstick is what helped me feel confident. And I feel like people could see that it's authentic to me."
Ten years ago, Kylie and her sister Kendall, now 22, were supporting stars in reality TV show Keeping Up With The Kardashians.
Their father, Caitlyn Jenner, was second husband to Kris Jenner, who has children Kourtney, Kim, Khloe and Robert with ex-husband Robert Kardashian.
Young Kylie was not afraid of the attention, though — and shocked viewers by spinning around a stripper's pole in the first episode of the E! show. Last year she also filmed her own eight-episode spin-off, Life of Kylie.
But panicking that her private jet had broken down and she might not make her prom on time, and telling a therapist of the challenges of living life on camera — while on camera — led to poor reviews.
When it comes to her love life, mum-of-one Kylie definitely has a type. She is dating rapper Travis Scott, dad to her five-month-old daughter Stormi.
Previous flings include rappers PartyNextDoor and Stitches. But her longest relationship was with hip-hop artist Tyga from 2011.
Kylie's transgender father Caitlyn was formerly known as Bruce, an Olympic gold-winning decathlete.
This year Kylie came under fire from transgender activists when she marked Father's Day in the US by posting childhood pictures of herself with Caitlyn, 68, before she transitioned.
But she has said she is very happy for her dad, adding: "I would feel guilty if I was the reason she didn't live her life to the fullest."
Not only has she blossomed into a mini-Kim in looks — with the pneumatic boobs and shapely bum — Kylie has built an empire worth almost three times the value of her older sister's $AU471 million.
Her success is based on a huge social media fanbase — 110 million followers on Instagram and 25.6 million on Twitter — keen to copy her signature looks.
There has even been a viral Kylie Jenner Lip Challenge, which involved people sucking air out of a shot glass to swell their lips.
Kylie started her make-up range with $AU337,784, earned from modelling and her Kendall + Kylie clothing label at Topshop.
She relaunched as Kylie Cosmetics in February 2016, selling her kits via e-commerce site Shopify.
Altogether, she has sold $AU846 million worth of make-up in just three years, making Kylie Cosmetics one of the "hottest make-up companies ever", according to business magazine Forbes.
She is pictured on the front of the latest edition with the headline America's Women Billionaires.
Loren Padelford, who runs Shopify Plus, said: "No other influencer has ever gotten to the volume or had the rabid fans and consistency that Kylie has had for the last two and a half years."
Kylie, who lives in an eight-bedroom mansion in Hidden Hills, California, is seen to be so powerful on social media that when she tweeted she was no longer using Snapchat, its stock market value plummeted by $AU1.7 billion.
On February 6 this year, she gained the most number of likes ever for an Instagram post, with 17.9 million fans admiring the first picture of baby Stormi. Kylie had somehow managed to keep her pregnancy secret. Inevitably, her shock announcement boosted her profile.
It hasn't all been smooth sailing. In 2015 she battled with Kylie Minogue over her bid to trademark their Christian name. Ms Minogue's legal team opposed the bid and called Kylie Jenner a "secondary reality television personality".
They settled out of court.
She has also experienced the negative side of social media and started the anti-bullying campaign #IAmMoreThan, which encourages victims to share their stories.
"Since I was nine, since I've been in the spotlight, that comes with so much bullying and attacking," Kylie said.
"Everything I do, there's a huge light on it. I'm OK with that because it is what it is, it comes with the lifestyle. It has still affected me a lot."
Coming from a family of self-starters, it is no surprise Kylie puts her work ethic down to her parents and siblings.
She regards Caitlyn as "a role model", and calls Kim "so inspirational".
But as the youngster heads towards billionaire status, perhaps she will be the one dishing out the business advice in the future.
