At a guess, I'd say they're around their late twenties with Bubs and Coach Z in their thirties and Senor Cardgage at least in his forties (ignoring them all being born around the same time, apparently). I'm pretty sure there's no real answer, though. The way the H*R universe works is purposefully nebulous, the only rule being that the characters remain themselves in everything that happens -- unless going out-of-character makes sense at the time and/or adds to the funny.
In-universe-wise, Homestar & Co. seem to pick and choose things from the "real world" and act them out in whatever way they like. They're like overgrown kids trying to play at being grown-ups half the time, who for some crazy reason (blessedly for us) have the freedom to do so in ways which may or may not make a damn bit of sense. Outside canon, it's whatever TBC -- and for the games, Telltale as well -- believes will work best towards delivering the funny while staying true to the characters. Those are my observations, at least.