Hayley Okines, who was also known as the '100-year-old teenager' because of a rare genetic condition progeria that made her age at eight times the normal rate, died on Thursday. She was 17.

Hayley's mother, Kerry, announced her death in a Facebook post. "My baby has gone somewhere better. She took her last breath in my arms at 9:39 p.m.," Kerry wrote.

Hayley lived at Bexhill in East Sussex. She was recently been treated in the hospital for pneumonia but was discharged from hospital on the day she died.

She penned an autobiography named Old Before My Time. In her book, Hayley described what it was like living with Hutchinson-Gilford progeria syndrome, which causes accelerated aging.

"The easiest way to explain it is it's like my body is 100 years old when I am actually 14. But I don't like it when people call me old, because I don't feel like I am 100 years old," she wrote.

She, however, said she wasn't worried about dying. "They said the Titanic wouldn't sink but it did, so that proves experts can be wrong and I want to prove the doctors wrong," she wrote.

Hayley lived years longer than many doctors expected after undergoing pioneering drug treatments in the U.S.

"Sometimes people ask me if I could have three wishes, would I wish I didn't have progeria. And I say no," Hayley wrote. "My life with progeria is full of happiness and good memories. Deep inside I am no different from anyone. We are all human."

The Progeria Research Foundation posted rich tribute to Hayley on its Facebook page.

"The entire Progeria family mourns together with many as we say goodbye to Hayley Okines, our smart, beautiful and spirited English Rose, who passed away today at age 17," the post reads. "Gone from our sight, but never our memories, gone from our touch but never our hearts - We will miss you."