I made a mistake during the production of NCFPB because I was rushing to write the character bios the day before the book was due at the printer, trying to get it printed in time for Origins that year. I thought I knew the dwarf age categories from memory, but I did not; I made Durkon way too young. I corrected it in the Adventure Game and WAXP, and I included it here just to make sure that it's clear.
Development-wise, Durkon is meant to be the dwarf equivalent of the rest of the Order: early to mid-20's. It's just that he's actually experienced twice as much life as that, and has a much higher Wisdom (that often mimics being older and more experienced).