Can manure be too old?

I am in the fortunate postion of having access to unlimited amounts of horse manure. The pile at the back of my friend’s stables is perhaps 20’ wide, up to 10’ back to front and over 4’ deep in the more inaccessible corners. It is black, odourless, dense and is easily sliced with a spade. The pile has been growing for years, decades perhaps, and a few of us quarry into it to enrich our garden soil. My simple question: is the older the better or might nutrients have been leached out over the years?

I’d welcome your opinions.