Thank you for contact UI Extension Forestry! Your trees are what foresters and the IRS consider "pre-merchantable" -- in other words, they have not achieved commercial value as potential sawlogs or veneer logs.
Therefore, you will need to grow them longer until they attain a minimum size of 18" dbh (diameter at breast height). Please feel free to contact your local IDNR district forester for an on-site assessment of your timber investment!
IDNR District Forester Map: http://web.extension.illinois.edu/forestry/pdf/tfb-nres-201-14.pdf