People hire people like them. It is not as much as the absolute age but the age group and difference in your age with rest of your peer group in the company.

If you entered a field at 35 (appear 35) when most of the peer group in the company is 21, you are going to have hard time getting hired for same role being performed by 21. If the company that expects/has most 40+ employees in management, you are going to have hard time staying in technical role unless you have built bridges with management that want to keep you around.