Brooke Greenberg, 20, is a genuine medical mystery: she's a girl who's barely aged, despite living for two decades.

Appearing on CNN's Headline News on Tuesday, her parents explained that she's stumped doctors so badly that they call her stunted development "Condition X." There's nobody else like her in the world.

She's so special that doctors have spent years examining her genetic code in search of an altered gene or gene sequence that blocks the aging process, thinking this little girl could hold the key to stopping it in others.

Her parents, on the other hand, don't have such far-off projections for their daughter and say they take things "one day at a time" with Brooke, who cannot walk or feed herself and still has the mind of a toddler.

"It's like having a baby," Melanie Greenberg said. "In one breath you're doing baby things with her, you're changing her diaper, she's obviously in a stroller. You tickle her and try to make her laugh. On the other hand, you realize that she's 20-years-old."

This video is from CNN, aired Tuesday, January 22, 2013.