Every season is ca. a anno and in episode "Soccer Mom in the Mini van" season 3 he says to Bones -"I'm only 35..."
And as a little exstra fact, Bones is 31 going on 32. There's 5 years between them.. In "the man in the mud" season 3, April, Sweets' girlfriend askes Bones what the age diffrence between her and Booth is and she says "5 years..."
That also means Booth was 33 and Bones was 28 in season 1
I don't think it was ever mentioned. I keep thinking 35, but i'm not sure. All i know is that him and Brennan are 5 years apart. So if te know how old Brennan is..he is either five years older o younger