I think it depends on the industry though. MC and IB are typically on the younger side. Industry, engineers, and folks like that are on the older side. Besides schools seeming to trend younger overall, I think its the economy and the type of people b-schools like to admit. Bankers are very popular with schools now look at that industry over the past few years. IBs hired heavily several years ago but the industry has been tanking so people are getting let go or are deciding that going to school is a good idea at the time. While a few years ago people would stay in the workforce because the banks were doing so well and it seemed pretty stable. So a lot of young people with highly desirable profiles are applying.
I tend to think that certain people NEED to be older to really have a chance. If you are an engineer, typically your first few years out of school are highly technical and specialized. You are doing the leg work on projects not so much leading things. Its the nature of the career. Engineers also are the group that most often already have advanced degrees.
If you didnt go to a ultra elite undergrad, get a high gpa, and then get a bluechip career for a few years, having more work experience will most likely improve your chances. I bet if you look at the people with less than 3 years work experience a much higher percentage of them will be Harvard, Stanford, UPenn, Princeton, Yale, Columbia...etc undergrads and will work for the more impressive name companies than the rest of the student body.
I really dont think its the age that is whats getting these people into b-school, I think its what their profiles are like. They may just be admitting more of these folks than in the past and with a 30%+ increase in applications over the last few years there are certainly more to choose from.