It's your age. We live in a very age-conscious society. The employer is looking at how many years you have left to retirement rather than what you have to offer the company in terms of experience, knowledge, and skills. There are some alternative methods of trying to secure a job.
1. Is your profession one where you can hire yourself out as a consultant or on a per-job basis?
2. Apply to companies that supply temporary help to other companies. If a company likes your work, everytime they need someone they will request you. Many times, it will lead to a permanent job somewhere. The company finds that they like your work and your personality; and, they decide to hire despite you being 50. Don't forget that places like Universities have their own temporary services; so, you apply directly at the University. They keep you working on campus only.
3. Do a bit of volunteer work. This can lead to a job from people who are able to view your work ethic.