Every piece of wood eventually reaches equilibrium moisture content (EMC). The percentage of moisture left in the wood at EMC varies only on the temperature and humidity of the surroundings where the wood is stored. Every piece of wood will reach the same EMC, regardless of species or age. A beam that is 60 years old is not dryer than the wooden floor that was placed three years ago or the bow that you built last year. So if the average temperature and humidity of the cellar dictate an EMC of 9%, it doesn't technically matter how old the wood is, as far as moisture content goes. 9% might be a bit on the low side for ash, but it's easy to move a nearly finished bow to a more humid room to elevate the EMC a bit.
In my opinion ash is a very poor choice of wood to make cellar supports from. Ash degrades very quickly by fungi, insects and bacteria. Ash in itself is a pretty decent bowwood, but I would inspect these beams very carefully for any signs of degradation. Perhaps do a bend test with a small left over piece. Watch for any pinholes, discoloration or crumbly physical appearance.