Who is the youngest Phd holder in world?

At the age of 20 Sergey Mergelyan earned his Doctor of Science degree in mathematics in addition to his Ph. D from Moscow Steklov Mathematical Institute. Up to today this is an absolute record of getting the highest scientific degree (Doctor of Science) at such young age in former USSR and present Russia. When he was just 24 he became corresponding member of the Academy of Sciences of the USSR (now Russian Academy of Sciences), which, from the point of view of young age, is yet another absolute record among USSR scientists.

(from Wikipedia).