A very odd question to me... (I mean, you can't really plan to be exceptional as a student and to be lucky enough to get a chance to conduct relevant research that is worth a significant publication.)
Happy reading; This should answer all statistical questions concerning Phds (not taking into account how useless such statistics are, aside from entertainment and dry academic politics).
[The answer is:
Tada! *drumrolls*
"Education" with a median of 17.3 years since baccalaureate. see page 59 on the document in the link]
