My oldest still in use is my Amazon account. Earliest order 2001. I did have a previous account but must have lost the logins as I did order from Amazon in 2000, and maybe 1999 too.
Email addresses have come and go. I've been on Gmail since 2006, which is the longest I've stuck with one.
I used to use newsgroups - but not for 10 years or more since forums have become common.