Having done the math (or tried to, anyway--Marvel timelines have nothing on DC, but are still screwy), I'm pretty sure he was about 26/27 then. All the Avengers have grown up a lot since those first days, though not as obviously as the X-Men, who all started out as teens rather than people in their early twenties.

Ah, Clint and Steve. I have a special love for the bit in Avengers volume 3 where Clint is "explaining" to Cap why he has no luck with women. It involves handgestures, posing, and "impressions" (Clint: "Gosh, golly, shucks, I'm Captain America. Watch me stand here and look noble." Cap: "Clint, the depth of your interest in my love life is starting to concern me.").