Actually, Pokemaster's answer is wrong. Surprisingly. Although there is probably some twisted logic behind this that I might need to know about.
metalmaster is not actually the first user on the site. (Except Pokemaster) Metalmaster joined on 26 April, 2010. But the person who asked the first question on the Pokebase (except for PM's test questions,) is actually the first user as well. (Again, except for PM's test accounts. xD)
Mewthree joined on April 20, 2010 making him the oldest user on the site bar Pokemaster.
Someone already figured this out on his wall, surprisingly.
Hope I helped. :)