The general rule is that employers are free to change an employee's compensation at any time. The terms of compensation need not be fair, provided they comply with the laws governing minimum wage and overtime, and provided that the employer is not motivated by a legally protected trait such as race or age in setting the terms.
It is actually not illegal for an employer to cut an employee's salary because they believe the employee makes too much or as a means of reducing costs. Companies do this all the time and it is unfair but perfectly within the bounds of the law. What would be illegal is if you could prove that your pay was being reduced BECAUSE of how old you are. However, candidly, the facts of your post do not indicate this is the case. In fact, in your post you state that the reason for the paycut was so that the company can reduce cost. Thus, it would appear their motive is financial and not related to age. You also do not state any specific facts to suggest that your employer has an age bias (i.e. they are not making derogatory comments about how old you are or paying younger less qualified employees more than you).
Certainly if you believe you can prove that you would be getting paid more if you were younger you can file a complaint with the Department of Fair Employment & Housing. That's the agency in CA that handles age discrimination complaints. However, if you do not believe you can actually prove that age is your employer's motivation, you would not typically have any recourse based on the facts you have described.
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