Grafted Carpathian walnuts start producing when they are between 4 - 10 years old depending on the variety. Even when given ideal growing conditions, seedlings take much longer. You didn't mention what sort of care these trees have received. If I were you, I would contact my local cooperative extension agent for help. You can give him/her more information and they should be familiar with any growing problems walnut trees in your area would have. Here is a link to the counties in Michigan. Click on yours to get the location of your nearest agent. http://msue.anr.msu.edu/county/
Bad news - I think Walnuts require a pollinator. If they were the exact same variety, then they are actually clones off the same original tree, and will not pollinate each other. Usually another tree in the neighborhood will work, but you may not have one close enough up wind. I have pollinated pears with one small branch from another tree hanging in my tree in a bottle of water. I don't know it that would work on walnuts, but it is worth a try. Start scouting around for other lonely walnut trees!