Caitlyn Jenner is ''so proud'' of her daughter Kylie.

The former Olympic athlete - who was known as Bruce Jenner before her sex reassignment surgery in 2017 - took to her Instagram account to praise the 21-year-old businesswoman on the success of her Kylie Cosmetics empire.

Alongside a photograph of Kylie's lobby, the 69-year-old star wrote: ''20 years ago, when your mom and I were changing diapers, teaching you to walk, and putting you to sleep, we had no idea this was coming. So proud. @kyliejenner (sic)''

Back in March, the 'Keeping Up With the Kardashians' star was named the world's youngest self-made billionaire by Forbes magazine.

The brunette beauty - who has 17-month-old daughter Stormi with Travis Scott - insists she ''didn't expect'' to be so successful when she launched her beauty business, which Forbes magazine values at at least $900 million, four years ago.

She said at the time: ''I didn't expect anything. I did not foresee the future. But [the recognition] feels really good. That's a nice pat on the back.''

Most of the brand's marketing is done through social media - where the brunette beauty announces product launches, previews new items and tells fans which shades she's wearing each day - to her 175 million-plus fan base across Instagram, Facebook and Twitter and Kylie believes it's been essential to her success.

She added: ''It's the power of social media. I had such a strong reach before I was able to start anything.''

Kylie has overtaken Facebook founder Mark Zuckerberg, who previously held the title of youngest self-made billionaire from the age of 23.

The make-up mogul signed an exclusive distribution deal in November last year with beauty store chain Ulta to stock her $29 lip kits - a matte liquid lipstick and matching lip liner - into the company's 1,000-plus stores across 50 states in the US, having previously sold only online or occasionally at pop-up shops.

Following her deal with the beauty company, Kylie Cosmetics sold $54.5 million worth of products in just six weeks, according to estimates from Oppenheimer.

Kylie owns her entire company, which she started in 2015, and has just seven full-time and five part-time employees, while manufacturing and packaging is outsourced to Seed Beauty and her mother Kris Jenner helps run the brand's finances and PR.