"According to the 2005 federal Survey of Earned Doctorates, the average age at which people earned their PhD in history was 35.3. The 2003 National Study of Postsecondary Faculty tells us that assistant professors had to wait an average of 3.3 years between graduation and the time they landed their current tenure-track position, which means they were, on average, 38.6 years old when they were hired as new assistant professors. "

http://phdinhistory.blogspot.com/2007/12/age-old-myths-of-history-faculty.html