I tend to read that wisdom as referring to things like mathematical theory being mostly advanced by people under 30. People's brains seem to hold onto some extra bit of IQ between 20 and 30, that can be used to do some "wondrous" things. Of course, most big things that need doing don't need a supergenius; they need 10-20 years of steady hard work, so by the time you accomplish those you are indeed 30+ (or, god forbid, 40+.)