Jimmy,
Dress as you like though the decor of hotel de la Montagne is a litle upscale (fancy chandeliers, fountains, polished grand piano, etc...). Some had suits, others golf shirts, just wear whatever you feel most comfortable in because there's no dress code (unless you want to hit Thursday's afterwards and get into the dance club downstairs...).
As for the age range, I'm 24 and though I was the youngest one there at the last meeting (except for the girls from the agencies who came), most of the MERBites there weren't that much older than me, some 30's, some 40's, but everyone was out to have a good time and you won't even notice a difference.