The WY Game and Fish just released tooth aging from the 2014 Bison hunt.
The average age of bulls was 4.7 years old. The median age was 4.5. (I didn't analyze the cow data)
Breaking down the age further, there were
2, 9.5 year old bulls
6, 8.5
5, 7.5
13, 6.5
27, 5.5
22, 4.5
9, 3.5
7, 2.5
5...