Brooke Greenberg turned 16 in January... but despite being a teenager chronologically, her body is that of an infant and her mind of a toddler.

And Brooke is alone... she is the only person in the world with this unknown syndrome.

Doctors are baffled.  There is no scientific explanation for Brooke's condition.  From ABCNews.Go.com,

Brooke hasn't aged in the conventional sense. Dr. Richard Walker of the University of South Florida College of Medicine, in Tampa, says Brooke's body is not developing as a coordinated unit, but as independent parts that are out of sync. She has never been diagnosed with any known genetic syndrome or chromosomal abnormality that would help explain why.

Brooke still has baby teeth, gets around in a stroller when her mom takes her shopping and her bone age is that of a ten-year old.  And her mom... tired of explaining Brooke's condition when asked how old Brooke is, just turns the years into months - and says, 16-months.

Brooke weighs 16 pounds and is 30 inches tall. She doesn't speak, but she laughs when she is happy, and she clearly recognizes the people around her. She has three sisters: Emily, 22; Caitlin, 19; and Carly, 13. All three are bright, active and of normal size and development.

She has not been without health problems and even took growth hormones.  But didn't gain one ounce or grow one inch.  Doctors are certainly mystified by Brooke's condition.  And scientists and doctors wonder if Brooke may hold some secret to the fountain of youth.

To hear more about Brooke Greenberg's story watch 20/20 on ABC tonight at 10:00 EST.  And watch the documentary airing on TLC on August 2, 2009 at 9:00 EST.