My understanding of MBAs was that they were of most value to people who'd achieved a certain level of seniority in one, or several positions. That experience could be brought to bear on the course and would not only enlighten you, but would help to enliven the course and keep it in step with what's going on in business.
To many who undertake business courses, it's an opportunity to break away from a particular career path and acquire wider understanding. In other words, an almost-fresh start. So wouldn't that make you as groomable at 40 as at 25? What's more, one of the greatest benefits cited by business degree-holders is the people they met on their course. Imagine how much more a diverse age and experience range would add to that benefit.
There are courses, too (the Sloane fellowship, for one) where experience is a pre-requisite. And older MBAs are different candidates and graduates, too -- more expensive, potentially, as they'll have more business experience to add to their theoretical knowledge. But it's not just pay, as Anburajan makes clear: she expects "more responsibility". And, if she's spent her 20s at work, you could argue she deserves it.
Business experience or groomability. Ideally, recruiters want both, but if you can only have one, which will it be, managers?