Female Wieder Spinnetods (Grimm) experience the effects of rapid aging since they are unable to molt without consuming human entrails. Charlotte, who's actually 26 years old appears 70 due to her choice to not partake in the tradition of eating entrails.
Jack (Bioshock) grew up into adult at age 4.
Due to planned genetic changes in his creation, Solid Snake (Metal Gear) was designed to rapidly age, appearing to be a man in his 80s despite actually being in his early 40s.
Martin Brundle (The Fly II), though chronologically five years old, has the body of a 20-year-old man.
As a Silpelit Diclonius, Nana (Elfen Lied) ages twice as fast as an ordinary human, appearing to be in her early-to-mid teens while actually being only six or seven.
Piccolo (Dragon Ball) became a full-grown adult in three years.
Upon birth, Adria (Stargate SG-1) aged rapidly until she was a grown woman within days.
Hiro-Kala (Marvel Comics)
Iris West II/Impulse (DC Comics) as a child...
... and an adult.
Jenny Quantum (Wildstorm/DC Comics)
As an E-type B.O.W., Eveline (Resident Evil) rapidly ages unless she is regularly given a special treatment to counter it.