The only real advantage a 20 year old has over a 40 year old is that a 20 year old body can survive insane working hours, skipped sleep, sleeping under the desk, etc. Also, they don't have families and kids yet. That's why some companies want 20 year olds.

Edit: I'm 39, and I'm finally hitting my stride. I'm a far better programmer now than I was even 5 years ago.