January and July. The summer pruning is shortening of new growth. Age of tree has no effect on timing. But more important is finding out how to prune before attacking the tree than when, exactly it is done.
Sure, I'd agree with January for "average". I prune them as early as the last week of October and prefer to do them before February is over. March is fine, but I'm not psyched for fruit trees as much from March onward. Likewise, I lightly prune in summer.