"I have usernames older than you."
IMHO, always good to remember that for any popular generational pass-time there was probably at least a microgeneration before who ironed out the kinks.
(That is to say, if the pass-time wasn't already popular before under some other name. Social media didn't start with X/Tumblr/whathaveyou.)
"I have usernames older than you" is a good way to bring this up. Though admittedly for every GenXer with a 1200 baud modem, there were ten with a nintendo.