Why is the age is one of the main factor to choose a candidate from an interview?

Because the fesh graduate's mind differs from the experieced one.

The fresh one is like a white paper, as a company what ever you want to do then you can. 

The experienced one knows what he should do, just time to be adapt to nature of the new place.

Because age should be align with job/work that a particular applicants shoud perform. As we all know there are some works that might not be fit already in an oldest candidates that required physical fitness.