Hmmm. Age is not a criteria per se, and I'm not sure camper was trying to say this anyway. I think he is alluding to what other experiences you can bring to the table in your application. Hence, a few years managing projects in the commercial world can be used to impress a potnetial supervisor who is trying to decide between 50 applicants with Distinctions in their MScs.
Obviously, many things are taken into consideration but gaining useful experience and transferable skills takes a few years so by implication age can come into it.
From personal experience, I don't entirely agree on academic ability alone. As I've postulated on here before I am just completing my UG degree but was able to successfully take on Masters grads with Distinctions because my years in the commercial world had given me a whole load of skills that the stereotypical Masters grad woudln't ordinarily have. And also because my proposal happened to be very good.
My point is that a whole range of criteria come into it, academic ability merely gets your application through the first round.