The youngster suffers from a rare disease called progeria which turns children old before they have reached their teens.
But now having celebrated her 14th birthday she has defied all odds and to celebrate she has published her autobiography.
Like any other teenager the book, called Old Before my Time, details pop-star crushes and a dislike of school.
But it is also a moving insight into how a child copes with a disease which has resulted in Hayley having the body of a 105-year-old.
The book has been co-written with the help of her mother Kerry who contributed to some of the chapters.
She writes: ‘She definitely seemed much wiser and more inquisitive than other 20-month-old toddlers.’
Progeria takes its name from the Greek word ‘proeros’ meaning prematurely old and involves a mutant protein called progerin that accelerates physical ageing.
It ages the body at eight times the normal rate leaving Hayley with skin that is thin and papery and bones that are fragile.
Progeria patients normally die from heart attacks or strokes at an average age of 13.
‘Mum says I am one in eight million because my condition is so rare,’ Hayley says in her book.
Hayley, of Bexhill, East Sussex, turned 14 in December and is among 89 children in 32 countries living with progeria.
Mum Kerry knew something was wrong when her baby wasn’t growing or putting on weight as quickly as other infants normally would.
Her family was left devastated when she was diagnosed with Hutchinson-Gilford progeria syndrome.
Hayley’s story was catapulted into the limelight after she starred in documentaries called The Girl who is Older than Her Mother and World’s Oldest Teenager: Extraordinary People.
In April last year, she was told by a doctor that her hip bones were in danger of dislocating and she now has to wear an uncomfortable brace nearly 24-hours a day.
But her family is hoping a breakthrough in science might help her live a long and better life than previous sufferers who have died of the disease.
Two U.S. girls are also taking part in the planned clinical trial in Boston which adds everolimus to the body which is believed to counteract progerin.
They are Lindsay Ratcliffe, eight, and Kaylee Halko, also eight.
Hayley said: ‘I feel like I have a future to plan. I think that maybe when I grow up, I will get married. Mum says it will have to be someone very special to see beyond my progeria.’
Last year Hayley’s mum told the MailOnline it was hard letting go of the reins as Hayley grows up.
She said: ‘It is difficult. ‘She obviously wants to go out and I do let her go down to the local shops and go on sleepovers.
‘She also goes to the local shopping centre with her friends but I like to be somewhere in the centre at the same time and I make sure she has her mobile on her in case she needs me.’
The internet has opened up a whole new world for Hayley, who loves the social-networking site Twitter.
It was thanks to her Twitter followers that she achieved a lifelong dream of meeting the pop singer Justin Bieber.
Hayley’s younger brother Louis and sister Ruby consider her to be a ‘celebrity’ because she’s been on Blue Peter and met famous people.
But she is increasingly aware that she has a terminal illness and has been shown laying flowers for a family friend with progeria who died at 14.
Her mum said: ‘We don’t sit down and talk about it but she’s had friends who have passed away and she’s aware,’ Kerry said.
‘We’d rather get on with living.’
‘I hope the new drug works out alright and Hayley stays healthy.’
Old Before My Time: Hayley Okines’ Life With Progeria is out now.
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