Stark has to be around 40.
In Ellis' run on Iron Man where Extremis was introduced, it was stated in a flashback that Tony was 19 during the Gulf War so Tony's probably about 35.
In Garth Ennis' Punisher MAX series Frank is nearing 60. In the current Punisher War Journal, I'd say he's portrayed more as very fit late 30s.
In some cases we can work out more or less actual ages for some of the characters, others are near impossible. In one of Dan Slott's issues of the Thing, Ben had a bar mitzvah, celebrating around 13 years of Ben being the Thing. I would say Ben and Reed are between 35 and 40 then, Sue early 30s and Johnny about 27-28, same as Peter Parker as I believe they're supposed to be about the same age. Are these the ages they're written at? Johnny for his age is very immature, as is Ben but in more of a "I'm a grown up so I can goof around" way.
Although this means that the FF debuted in about '94, but Tony's retooled origin (taking place in the Gulf War instead of the Korean War) would then take place 3 or so years before the FF, but Iron Man debuted after them in continuity. Confusing and irreconcilable.
There are hundreds of other characters I could go into regarding age and the paradoxical affects that their ages and portrayals take on continuity.
As an aside, I've always felt it was tragically humurous that because of our present society's cyclical nature of war, just as many character's origins start to become outdated, we can simply change the conflict in which they were birthed. Tony Stark went from the Korean War, to the Vietnam War, his origin date lingered in limbo for awhile and was recently updated to the Gulf War. Same can be done for Punisher if need be, Jim Rhodes, Red Wolf and others can be similarly updated. Give it another few years and they're origins will be keyed into the present day conflicts in Afghanistan and Iraq.