I spotted this thread this morning so I did some search using Scott's tips and... putter2eyes is probably the oldest active player.
This fella and IRISHKING1916 both joined WGT in June 2007, but his user ID suggests that he joined tad earlier than Karl did. (The user ID of putter2eyes is 30,585 and Karl's being 33,527.)
This search would've never been conducted if it wasn't the COVID-19 confinement.
Be safe and well, everyone.