I got an invite to Gmail when I was in university. Sadly I was at inbox zero for quite some time so I don’t have the Welcome to Gmail email anymore. It’s more than likely my oldest active account. I do have a Neowin account that still works from 2002 but I don’t frequent anymore. Macrumors is my most active old forum account, that’s from 2005. I had an ancient AIM and ICQ account but there’s no way I could remember the account details.
I mean you could always make a new account to get the welcome email and photoshop the dates and times to make it look like it's older. I imagine the look of the email now is different from 2003, tho.
I still have a Slashdot account from 2009. Haven't been on there much in years though. Google would probably be my oldest actively used account as I still use the one I set up with the initital invite.
Shiiit, actually that does make sense. I remember when Gmail gave you 10 invites for new accounts and you didn't even get them right away. You had to earn them over time. So yeah, invites were regularly sold on the message boards I used to frequent.