The Jargon File (version 4.4.7, 29 Dec 2003):

old fart
 n.

    Tribal elder. A title self-assumed with remarkable frequency by (esp.)
    Usenetters who have been programming for more than about 25 years; often
    appears in sig blocks attached to Jargon File contributions of great
    archeological significance. This is a term of insult in the second or third
    person but one of pride in first person.


The Free On-line Dictionary of Computing (30 December 2018):

old fart

   Tribal elder.  A title self-assumed with remarkable frequency
   by (especially) Usenetters who have been programming for
   more than about 25 years; often appears in sig blocks
   attached to Jargon File contributions of great archaeological
   significance.  This is a term of insult in the second or third
   person but one of pride in first person.

   [Jargon File]