Greetings Toby:
Depending on certain factors -- namely site, soils, maintenance, intermediate stand management, planting stock, etc. -- you certainly could have some merchantable black walnut trees after 35+ years of growth. Obviously, you should contact a local professional forester for an on-site evaluation with regard to this walnut plantation.
http://web.extension.illinois.edu/forestry/forest_mgmt/professional_assistance.html