De-aged crew members
Preadolescence was the developmental phase between childhood and adolescence, typically occurring between the ages of 8 to 12 years old.
In 2369, Jean-Luc Picard, Ro Laren, Guinan and Keiko O'Brien were physically reverted to twelve-year olds after the transporter deleted rybo-viroxic-nucleic sequences from their genes. Later, Doctor Beverly Crusher brought up the results by clarifying, "According to the bioscan, their DNA structure is now consistent with those of preadolescence." (TNG: "Rascals")