This behavior has nothing to do with sex or with age. He is trying to dominate you. He does not respect you as the leader of the pack. No dog of any age would try that with the pack leader.
Use you hand as a mouth and when he exhibits any agressive or unwanted behavior, nip at his shoulder and say no or stop or don't or whatever word you choose. But don't let it continure.
It may be funny to you, but the longer it continues, the harder it will be to correct.
check out this website for more information.
http://www.dogchannel.com/dog/expert...aspx?WT.srch=1