What, the sky falls in if Hawkeye is over 30?
You have a nce list of completely circumstantial evidence there - I usually just go by how old Hitch draws him to look, which is usually early- to mid-thirties but occasionally closer to 40. Somehow he looks oldest in full military uniform, youngest in V2 #7, lounging around the house in a tshirt. It's not very consistent, which goes along with how old his kids are - the V1 they all look like they could be somewhere around ten, with the oldest just about to hit the teen years. In V2 #5 they look they are in between about 3 and 7, yet in the latest issue the youngest can be inferred to be either an infant or very young.
Jan looks like she is in her late 20s, the Widow looks somewhere between 25 and 35, Tony is definitely in his early 30s, as is Cap. Hank is another one that changes age, but I'd peg him at least late 20s if not early 30s. Banner always looked younger than the rest and confused, mid 20s at best (which is inconsistent with how old his character HAS to be, but we'll let that slide). Fury is at least in his 40s and maybe late 40s. The twins... mid to late 20s, I'd say. Thor most often looks sort of timeless, that whole "epic hero" thing. He could be 25 or 250.
All of that depends on the issue, the angle and phases of the moon, but there is usually a shot I think of when I think of a certain character.