Browsing W3Schools, I've found something interesting – IE6 is older than Netscape 4.
Sort of. From their incomplete and somewhat dogmatastic overview of web browsers:
Netscape Communicator 4.8 was released from Netscape in August 2002. whereas
[Internet Explorer 6.0] was released in August 2001.
I know this has nothing to do with the age of the rendering engine, but dumb users know and care nothing about the rendering engine; in their eyes Netscape 4.8 is newer than IE6.
So if you're gonna stop supporting one of them next time you design a website, shouldn't it be the oldest? (Ideally, screw both of them.)