My observation on major airline dispatch is that it's less of a career path, and more of a right-place-right-time kind of profession. I remember a conversation I had with a Delta dispatcher. He had 25 years in with Delta, including supervisory roles, BEFORE he moved into Dispatch. Now, Delta is kind of the outlier in that regard, but when you see that American, Southwest et al, give just as much preference to internal candidates with no experience, as they do to external candidates with experience, it makes it kind of hard to plan a career. This is anecdotal of course, but my perception is that the majority of my colleges over the years DID NOT make it to a major. A lot of them have, but a lot more of them haven't.
With regard to age; it's been my observation that it's not so much a factor as professional growth (or lack thereof). In other words, it doesn't matter if your 50+. It matters if you're 50+ and spent the last 25 years working the same desk at the same regional. I think the underlying bias is "why has he been slogging away for 25 years with no advancement?" If you just sit on the desk and wait for the majors to come calling, you may be sitting there a white. You should be constantly striving to develop your career. After 2 or 3 years of experience, you better be showing some growth. A more responsible position, a bigger company with bigger planes, something. I had Domestic, Flag, Supplemental, Passenger, Cargo, Instructor, Supervisor experience, and dispatched everything from Dash 8s to 727s, and it still was 10 years before I got an offer from a major.
This is why I get a little frustrated when I see the "it doesn't pay much, but it's good experience" mantra. Yes, you need experience, but you can't trade money for experience forever; At least, not if you want to have some kind of normal life-arc (spouse, home, kids, etc). As I said earlier in the thread, the dispatcher's role at the regional has changed; dramatically. The compensation has not.