Oh, some industry blowhards and deep thinkers will dance around the issue, rejigger their definitions and come up with a million ways of explaining how, gee, it's just a coincidence that somehow middle-age IT professionals so often lose out in a job hunt to kids in their 20s. But the statistics don't leave much doubt. That's too big a gap to be explained away by bad luck or happenstance. Put simply, lots of IT managers believe that if you're over 40, you've got a 20-year handicap compared with programmers, network administrators and help desk workers just out of school. Younger IT workers have fresher, sharper technical skills than middle-age IT people, or so the justification goes.
Younger workers have the energy to put in longer days.
Younger workers aren't distracted by having social lives and families. Some of those arguments are bogus. Lots of older programmers and network managers keep up with the state of the art. If you don't understand all the technology options, you can't map technology to business needs. But some of those justifications are, well, justified.