UPDATE: Thanks to Andrew for pointing out my mistake about Harry's two possible years of birth (1987 and 1977, not 1997 and 1987). I've now corrected them here.
Just noticed something about an ad for ASM #581 (which can be found in Big Hero 6 #3 and, I suppose, elsewhere): The page (cover to #581) prominently features the gravestone of Harry Osborn, and the year of his birth is shown ending with the numeral 7 (you also see the first numeral, 1, but the 9 and whatever number follows are obscured by Spidey's right hand). So Harry is supposed to be either 21 (born in 1987) or 31 (born in 1977), and Peter, Liz and Flash, who all graduated from high school the same year as Harry and Gwen would be about the same age. Twenty-one clear is not possible, Peter was that age when he graduated from college (four or five years after entering ESU, assuming he was 17 at the start (1), gives you 21) thirty years ago (readers' time), and after that there has to be time alotted for Peter's post-grad studies (at least a year or two) and for Liz's and MJ's pregnancies (at least two more years (2)). So even assuming Peter is two years younger than Harry (3), he would now be approaching thirty years of age...
(1) Flash was drafted into the army not that long after he and Peter started college, and he'd have to have been at least 18 then.
(2) Leaving aside the question of the interval between them, which, judging by the age-difference between Normie and Mayday in MC-2, should be a couple of years at least.
(3) Which would entail assuming that Harry was such a bad student that he took a year longer to finish high school and Peter was such an exceptional student that, even though it never was mentioned before, he skipped a grade and is actually at least a full year younger than Flash and Liz.