considering the attrition rate of the average sharty user, i would classify an oldfag as "anyone who has used the site for 2+ years" - that is, if you joined at the start of 2023 you would be an oldfag now. and in 1 more year, joining at the start of 2024 would be considered oldtroon status
but really i don't want the soysphere to become one big "who's the older oldfag" circlejerk like what happened to the 4cucks, who im pretty sure are still obsessing over how anyone who joined after 2012 is a heccin' newfaggerino (2012 was 13 years ago...) while creating nothing but gooning generals