ADEA(age discrimination of emplyment act) from 'http://www.aarp.org/money/careers/jobloss/a2004-04-28-agediscrimination.html'

Age Discrimination in Employment Act
The Age Discrimination in Employment Act (ADEA) is your first defense against age discrimination. There has to be a lawful reason - not connected to age - for almost all employment decisions.
Who Is Covered by the ADEA?
The law covers workers and job applicants age 40 and over.
The ADEA applies to employers with 20 or more employees. This includes local and state governments and the federal government. It also includes employment agencies and labor unions.
The ADEA does not apply to independent contractors or elected officials. It does not usually cover police and fire workers, certain federal employees in air traffic control or law enforcement, or certain highly paid executives. While persons in these positions could be retired on a mandatory basis, they cannot be denied a promotion or training base on age.
There are exceptions to the ADEA when age is a necessary part of a job. For example, an employer can hire a young person to play the role of a 12-year-old in a play.
Most states have anti-age discrimination laws that apply to employers with fewer than 20 employees.
What does the ADEA forbid?
Job ads or recruitment materials cannot mention age or say that a certain age is preferred.
Programs cannot set age limits for their trainees.
Age can not be a factor in making any decisions about workers. This includes decisions about hiring, pay, promotions, or layoffs.
Employers cannot take action against workers who file a charge of age discrimination or who participate in any ADEA process.
With a few exceptions, employers cannot force employees to retire at a certain age.
Employers may offer voluntary early retirement without violating the ADEA. However, these offers often require employees to give up their right to make a claim under the ADEA. That requirement may be legal, but only if it follows strict rules. Get expert advice before you sign anything.