I am at least 13 years old
Google have extended their archive of Usenet postings back to 1981. Thus I have found evidence of myself posting during my summer placement at Hewlett-Packard in 1988, and as a graduate student (briefly) at the PRG (as it then was), and offering advice on HTML usage in February 1994 (before HTML was fashionable!). But that is probably enough ego frenzy for now.