Rather than rely on stereotypes here, I think one way to get some decent info about this would be to head over to the BusinessWeek Business School section and to click through the profiles for each school. (Link: http://tools.businessweek.com/BSchool_Comparator/
) On the first page, if you scroll down, you'll see average age and/or average months of work experience (in some cases, lower and upper deciles, too) of the latest admitted class (2011 grad year). If you pull that into a chart in Excel, you'll get a decent picture of which schools were friendlier to younger applicants in the last admissions cycle (before this year's). The flip side of this method is that you wouldn't know how many students of a given age applied to the school in the first place -- so the age ranges could be a function of who would like to attend the school more than who the adcom prefers. There's probably a little of both involved.
In general, though, most, if not all, adcoms will encourage younger applicants to apply but may have specific instructions for younger applicants in terms of how to best present themselves (Chicago Booth, for example, has very specific recommendations for younger applicants on their prospective students' website).
All that said, I do believe Harvard and Stanford have notably younger admitted students than most of the other top 10-20.