I joined right around the beginning of the launch. When he was actually working on the forum. Wasn't more than a month old. The DB was imported from another site he owns.
Assuming Omni was the first member : 06-22-2007 ,so 3 years old .. but i could be wrong,just ask omni if this is really that important to you :-)
EDIT:Or maybe not,2009 :-)
Yo stewie, omni already gave the answer, right above your post :/
Omni's way of identifying how old the forum is wasn't as accurate though, You'd have a better bet checking uid 1's registration date (Omni) because that user gets created on the original MyBB Install and also added to the list of super admins in a config file.
I think he intended that to serve as an estimated date for the creation of this forum.