As far as NY, I did notice at the Golijov festival last spring, at the Rose Theater in the Time/Warner building, a greater percentage of somewhat to much younger people in the audience than I generally see on programs at the NY Philharmonic which include a new piece. This is also true of Alice Tully Hall. Perhaps younger types feel more comfortable in smaller halls? Although the Rose Theater is quite large, it isn't a barn like Fisher.
Most, if not all, of the Grendel performances at the NYS Theatre were sold out. NYC Opera's new operas generally get a youngish crowd.
In Santa Fe, the new operas also get a younger (& less affluent) crowd than the traditional operas. This is also true of the SFe Chamber Music Festival for new works (& dance as well) at the Lensic Theater downtown.
So I guess I have to conclude that I don't see this as so in my two home cities.