It's funny you ask this question, as two days ago there was a special on TV about Whitetail Deer that stated that there life expectancy is in excess of twenty years, if they were to die of old age! However as we know because of predators, hunting, cars, etc few will make it that long.
That put something in to focus for me, if a humans life expectancy is 80 years and a deer's is 20 that would mean they age at an equivalent of four years to our one. In that case a six year old deer is equivalent to a 24 year old human. Considering that, I question if they are really going to be tough as we are lead to believe.
In ten years of processing venison we have had deer of several age groups and in all that time I've only had complaints on one large buck that was shot in Ontario.
This leads me to believe that geography and diet are greater factors than age in affecting the quality of the meat. If your deer came out of "big woods country" then you may be right, however if it came out of the agricultural / wood lot areas I doubt if it will be tough.
Try some of your venison before making any judgments. If it is indeed tough then try some of these solutions.
One customer always had all of his steaks run through the cuber once, and claimed that made them tender.
Vacuum tumbling with a marainade helps.
Use a commerical meat tenderizer. (Adolph's meat tenderizer is good.)
Brown your steaks then simmer in a sauce (Swiss Steak style)
Crock pots are always a good idea with tough cuts.
If all else fails grind it. Ground venison is still the most versatile of all.
Dave & Ruth