Part of the reason for this is to avoid any possibility that they can be sued for age discrimination. I was part of a 10% of a 600 person company layoff (based in California, but with offices in Europe and Asia). We all got a list of the people laid off (without names) with job title, and age. This was apparently a legal cover-your-ass requirement. They made sure to spread the pain across the young and old.

If they just got rid of the very high level (read: expensive) employees, they would likely be older - which can lead to age discrimination suits. If they just got rid of really junior people because they're so junior that they aren't that valuable yet, that's also a potential age discrimination suit. So they spread out the seniority (and ages).