I think any adult can experience a 'crisis' regardless of age -- i.e., anyone can suddenly get the feeling that their life has no real meaning or purpose, or they can't find motivation to get up in the morning because everything seem futile.
I'm no expert, but I think loss of purpose can be triggered by a wide variety of life events, ranging from the loss of a spouse or child, to finally achieving a long-term goal, to something as simple as feeling that you've been left behind by your peers (we seem to get this feeling most strongly after a high school or college reunion!).
It also seems that people who experience mid-life crisis seem to get younger these days -- which is kinda ironic since our life expectancy in general is improving. I guess that's why I now hear of quarter-life crisis (having a crisis in your 20s).