It will still have nutrients left in it. I've used "old" horse manure from a stable that was at least 4-5 years old and the plants performed beautifully.

A note of caution tho with horse manure. Unless it has been hot enough to kill any seeds that have passed through the horse's digestive tract, be prepared for unwanted weeds. Horses are notorious for inefficient digestion which is why you see so much green and grain in the road apples that are fresh. If you are getting from a stable, unless the pile is deep and self-heating hot enough to kill organisms or seeds be sure that you are on top of anything unwanted sprouting. I've had things like thistles, pigweed and other nasties sprout. Especially pigweed. For adding organic material to your soil, it is excellent. Mulch well.