Nobles destined to remain in a child like state maturing only to a certain point, or Nobles who continue to age despite being Noble. According to the physician who attended his birth, such Nobles are born from time to time.
Some examples are Alice and her daughter Lady Ann who are about 10 looking but are hundreds or thousands of years old.
Others are Chancellor Cornelius, and many of the Privy Council Ministry. Cornelius use a Eutrophic Fluid Containment to maintain his body due to it's advanced age.