I'm sorry to hear of the situation.
You seem, by your facts, to be confusing several laws regarding employment.
When the employee resigned---while there may have been a promise by the employer to rehire the employee when they were recovered---absent consideration for the promise---making it an enforceable contract---the promise is not enforceable under the law.
The fact that they employee may have been, or is, disabled, in no way impacts on the obligation of the employer in this situation. Neither does the age of the former employee---unless the employee can affirmatively prove that the reason they are not being hired is because of their age or disability.
If that is your position, then you would have the right to file a discrimination claim.
CA law prohibits harassment and discrimination in the workplace and you do have a legal remedy.
Workplace harassment/discrimination is any unwelcome or unwanted conduct that denigrates or shows hostility or an aversion toward another person on the basis of any characteristic protected by law, which includes an individual's race, color, gender, ethnic or national origin, age, religion, disability, marital status, sexual orientation, gender identity, or other personal characteristic protected by law. A conduct is considered unwelcome if the employee did not solicit, instigate or provoke it, and the employee regards ***** ***** as undesirable or offensive.
In CA you have two possible avenues of approach to dealing with discrimination. If your goal is to ultimately sue in Federal Court, then you will file a complaint with the EEOC, and if you want to be in the CA Superior Court---local to your county---then you will file with the DFEH and, if you want to, with the EEOC as well.
You may a formal complaint with the CA Department of Fair Employment and Housing alleging discrimination based on your age or disability status.
To do this you must first make an appointment with the Department to be interviewed, either over the phone or at a local DFEH office. You may call the DFEH at(###) ###-#### or apply on line by using the Department’s "Online Appointment System." The system will guide you through questions to determine whether an appointment is right for you.
Alternatively, you may file a complaint with the EEOC (Equal Employment Opportunity Commission). If your company has 15 or more employees, they are prohibited from discriminating against you. To file a complaint with the EEOC, contact the nearest Equal Employment Opportunity Commission field office. To be automatically connected with the nearest office, call(###) ###-#### EEOC website: www.eeoc.gov
Federal law specifically prohibits discrimination, based upon the Ethnicity, Color, Religion, National Origin, Age, Sex and Disability of an individual, with regard to hiring, promotion and firing.
After you file the complaint, your employer will be prohibited from any retaliatory action against you. The EEOC will investigate your claim, and 180 days after the filing of the complaint you may ask for a "right to sue letter". The EEOC will issue you the letter which gives you the right to institute a private civil action against your employer and seek monetary damages.
I hope you found my answer informative, even if the law is not necessarily in your favor. Please Accept my answer so that I may be compensated for assisting you. I wish you the best in 2012.