You can't really generalize on or off-B. It has a lot to do with the specific show, and in the case of off-B companies, their programming determines their demo.
A show like Hand to God draws younger on Broadway.
The crowd at the Public, NYTW etc almost always seems heavily skewed toward the student debt crowd whereas at RTC and MTC and LTC, it veers toward the Social Security set. Shows like Mercury Fur and Gloria draw young people, shows like Shows for Days and Informed Consent don't..
I think Craig is right to an extent, but I think it has more to do with the choice of play than venue. Will Ivo draw young people to Broadway the same way he did at NYTW? My guess is no, but there was not a sea of gray hair at the Rylance Shakespeares or, for that matter, Jerusalem. It's interesting that more young people are interested in Rylance than the superstars the older set shells out money for.
But as I say you can't over-generalize. What Craig has to do is choose plays that appeal to the younger theatre goer, not just do it off-B.