Thank you very much for your reply and clarification.
The general rule in California is that, absent a specific agreement to the contrary, employment is "at will," and as such, the terms of employment (including compensation) can be changed at any time, regardless of whether the basis is fair or reasonable.
The one very impotant exception to the above principle that is relevant to your concerns is that an employer cannot take any adverse employment action against a worker--such as a pay reduction--for reasons relating to age. So, if you can prove that your bonuses are being cut due to an age bias, that would be actionable.
The problem with alleging age discrimination is that is it very hard to prove. The U.S. Supreme Court recently raised the evidentiary threshold for these claims by ruling that a plaintiff alleging age discrimination must prove that "but for" their age, the same adverse employment action would not have been taken. (See Gross v. FBL Financial here: http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&navby=case&vol=000&invol=08-441
Basically, an age discrimination claimant can no longer prove that age was one motivation of several--they must prove that their age was the driving force behind the adverse employment action above all other things, and they must prove this by a preponderance of the evidence.
What an age discrimination claimant would would want are comments that derogatory about their age (i.e. that they're "too old to cut it," or have "stale ideas"), a culture of favortisim toward younger employees, or poor comparative treatment of other older employees. This is the sort of evidence--preferably more than one of these things--that juries look for in these cases. Mere speculation that age was the reason for the adverse treatment is insufficient.
In your case, none of the presumably younger employees had their bonuses cut, and unless there are legitimate business reasons for this decision, it certainly makes age discrimination a plausible explanation. However, an individual in your circumstance would be wise to give some consideration as to whether there may be additional evidence of discriminatory intent, such as in the form of the examples provided above. Assuming your boss cannot articulate a legitimate reason for cutting only your bonuses, you certainly have a case for age discrimination, but the strength of that case would be bolstered by this sort of additional evidence.
In short, although employment is ordinarily "at will" and thus subject to a change of terms at any time, employers cannot take adverse action on the basis of an employee being more than 40 years of age. Given that you are the oldest employee and the only one whose bonus was cut, that certainly raises a question of age discrimination and potentially forms the basis for a claim. However, additional evidence of discriminatory intent, which can take a variety of different forms, would be what you would ideally want to have a truly "strong" case.
If you are interested in proceeding forward with a claim, you would be well served to locate and consult with an employment law attorney in your area. Most attorneys handle this sort of work on a pure contingency fee basis. If you don't know, a contingency fee arrangement is one in which the attorney receives a portion of the client's settlement or award as his payment, typically 1/3 of the total amount. If there is no recovery, the attorney does not get paid. The client never pays until the settlement or award is obtained (except perhaps to cover the filing costs for his claim).
To locate an attorney close by, see here: http://www.cela.org/?page=4
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