Scientists wonder if Brooke Greenberg, now 16, will help point the way to new discoveries about the genetics of aging. Pictured from left to right on Brooke's 12th birthday are sister Caitlin, 15; Brooke; sister Emily, 18; mom Melanie Greenberg; and sister Carly, 9.
"We love her just the way she is," said Melanie Greenberg. "We don't want to change her."
Watch Brooke's story on "20/20" FRIDAY at 10 p.m. ET. (Courtesy Greenberg Family)