At least here in the states, northern state grain crops are sown in the spring and harvested between July and September, depending on where it's grown. Southern states typically plant in the fall with harvest falling between May and June. Theoretically, late summer and fall would likely be the most opportune time to get fresh grain.
It's probably hard to say that there's an optimal time to buy, simply because you have the growers, maltsters, distributors and finally the vendors that are involved in the distribution chain. All of these companies are going to work on the first in/first out principle in order to maintain the highest degrees of freshness for their products. If any one of them are trying to get rid of older stock, you could probably be stuck with older grain regardless of when you buy. The best advice would likely be to purchase from a vendor that rotates their stock frequently, such that you know it's at least fresh to them.