The oldest cheddar I've had was 17 year old, in Frankenmuth Mi. I also tasted the 15 year aged cheddar just moments before. I enjoyed the 15 year old cheese, but not so much the 17 year aged version. It had more of a "ripe" flavor, if you know what I mean. The 15 year aged cheese still tasted like cheddar, but with an undeniably complex flavor, and that mineral graininess that begins to develop after three years of aging.
I will be anxiously awaiting a review of the 40 year aged cheddar.
Seeeeeeeya; Chief Longwind of the North