Can an employer deny me of insurance benefits because it will cost too much?

I started a new job (in the state of California) over 90 days ago. When I started I was told that I would receive medical insurance benefits after my 90 probationary period. My probationary period is now complete and the office manager began to make arrangements for my medical insurance coverage. A few days later I was told that the company would have to pay $700 a month for my coverage due to my age (54) and that they wouldn't pay that much because all other employees (that are 1/2 my age) only cost a couple hundred dollars a month. And that if I wanted the insurance I would have to pay the $500/mo difference. I don't want to jeopardize my job in any way my suggesting that I feel I am being discriminated against due to my age, but I don't feel that it is fair that 100% of other employee coverage is being paid for and because of my age I have to pay for mine??? If there are any suggestions on the legality of what my employer is doing or suggestions on how to approach the situation it would be greatly appreciated. Thanks!