Syndrome X

Layla Qualls looks like a happy, 9- to 10-month-old baby— but she’s actually three years old. The Oklahoma girl was diagnosed with Syndrome X, which means she will never age.
Researchers from the University of California Los Angeles studied Layla as one of seven children found worldwide with Syndrome X to understand the extremely rare condition, News9.com reported. They hoped to find the condition’s genetic blueprint to see which genes are slowing down the aging process.

http://www.foxnews.com/health/2015/05/08/oklahoma-girls-rare-syndrome-prevents-her-from-aging/?intcmp=ob_article_footer_text&intcmp=obnetwork

A woman who died last week at the age of 20 never aged.

Brooke Greenberg, of Reisterstown, Md., puzzled medical professionals for years because she still looked and acted like a toddler. Doctors eventually dubbed her extremely rare condition "Syndrome X."

“While the outside world may have noticed Brooke’s physical stature and been puzzled by her unique development state, she brought joy and love to her family,” Rabbi Andrew Busch, who delivered the eulogy at the funeral, told the Daily News Monday.

“Her parents, three sisters and extended family showered her with love and respected her dignity throughout her entire life.”

http://www.nydailynews.com/life-style/health/woman-dies-20-toddler-article-1.1499031

See also:
http://en.wikipedia.org/wiki/Brooke_Greenberg