Answer:
67 years old.
I depends on who you think the Zodiac was. If you go by the bestselling book by Robert Graysmith then the Zodiac would be 77 years old. Arthur Leigh Allen, born in Hawaii and lived in Vallejo, California at the time the murders began.
Recently there was DNA testing on some envelopes from some of Allen's correspondence but it did not match Allen's. But there was no proof that he licked his own envelopes personally. Allen had a nearly genuis IQ, and although DNA was not possible when the Zodiac crimes were commited, Allen was smart enough to not put anything of a personal nature that could in anyway lead back to him.
After Graysmith's book was published, police were getting ready to bring him in when he suffered a fatal heart attact. Graysmith puts out a pretty strong argument for Allen being the killer. In fact, in his research he found that Allen used to visit Southern California on a regular basis, and his visits south align with several murders around L.A.