Fair question: It's less about why someone "deserves a job" and more about human rights. We are all human beings who deserve equal treatment under the law, including consideration for employment. Older people breathe air and eat food just like everyone else. If it were legal to exclude +65 (or +40) from employment, what exactly are they supposed to do?

This fairness argument is complicated by what happens in down economies. Experienced (read: older) people tend to crowd out the younger folks for jobs. I don't know an equitable fix for this, but throwing people out of the workforce for something they can't change doesn't seem right.