Earth-616 is on what's called, a sliding time scale, which basically means that most of the characters existing today have only aged a few years since their inception in the 1960's. So it's difficult to say with any accuracy how old any character actually is. Without the sliding timescale, Peter Parker was about 15 in 1963, and would be 61 today! With that in mind, Hulk originally appeared in 1962, and was an adult at that point. If we conservatively estimated his age then to be 30, that would make him 76 today.