It's not the salary, as much as where you are in your current career, and what your post-MBA plans are - especially for full-time programs.
Keep in mind that full-time programs are designed for those who are relatively early in their careers and the kinds of jobs you'll get coming out of b-school are still entry level type of positions (i.e. you are basically one rung up from a fresh college grad in many cases). As such, they want to make sure that your expectations are realistic coming out of b-school given your background - are you willing to take a significant step back in terms of salary but more importantly responsibility to switch careers or job functions? It's easier to do when you're younger and make less money - there's less to trade off. But as you get older, more and more people are less willing to do that. If you can convince them though that you understand the compromises and tradeoffs you're making, then a lot of top schools (save for HBS, Stanford and Wharton) are more willing to take your word for it.
Also, keep in mind that since most of the post-MBA positions are entry level type of positions, or at best the lowest level of a middle management position, recruiters tend to favor younger (they're not supposed to discriminate, but for a lot of post-MBA positions, they really prefer the fresh young meat many of whom are more willing to sacrifice for their careers -- those in their 30s tend to be less inclined to do that).
In any case, I've addressed this in my blog in more detail at
http://mbaapply.blogspot.com -- rather than reposting everything here.
Alex Chu
alex@mbaapply.com
www.mbaapply.com
http://mbaapply.blogspot.com