My oldest one is from 2006, that's when I set up Thunderbird for my gmail account.
Actually I cheated, cause I have no mail in my inbox, all the mail I recieve is directly moved into folders, or deleted.
And than if it's a folder not for family or friends, I have set a retention policy for them (like newsletters and such). And about once a month, I log in on the web, and clean that out.