The toughest elk I ever ate was a big cow. Since I have NEVER killed a really MATURE bull (lol), can't speak for that, but the bulls I have killed are OK to eat. What I found is aging is VERY important for elk. The cow that was so tough, I killed one day and it was cut, packaged and frozen the next day. I learned to let them hang at least 3 days (4-5 is better) and it makes a world of difference in toughness. Marinading overnight makes a difference too.