Aging of the field emission performance of the printed carbon nanotubes (CNTs) cathode is studied. A continuous increase of the field emission current as well as the density of field emission sites under a constant voltage is observed. It is revealed that the resistant heating may play an important role in the activation of the potential emitters during the aging process. A technique of activating the printed CNT cathode with an aging process is suggested. F-N curves before and after the aging processes are analyzed and it is revealed that the effective emission area increases during the aging process.