It is possible, because a couple of months ago I reactivated (with help from Linden Lab) this 2006 account which had been dormant since 2007. But you will have to know something about the old account. In my case I did not know the password or the email address but I gave my name, address, date of birth (all of which I had provided truthfully when I registered), a few names on my friends list, and a bit of billing information that they were apparently able to use. I did it all through a support ticket, I didn't have to call them and it took less than 24 hours.
I facepalmed when I logged in and realised that the email address on file was one I still have, so I could've simply done the "forgotten password" thing after all.
Everything in my inventory was still there. Tons of 2006 freebies.