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The Age Discrimination in Employment Act of 1967, Pub. L. No. 90-202Code, 29 U.S.C. § 621 through 29 U.S.C. § 634 (ADEA), forbids employment discrimination against anyone at least 40 years of age in the United States (see 29 U.S.C. § 631(a)). The bill was signed into law in 1967 by President Lyndon B. Johnson.
The ADEA includes a broad ban against age discrimination and also specifically prohibits:
Mandatory retirement based on age is permitted for:
Written in 1961, the Age Discrimination in Employment Act of 1967, Pub. L. No. 90-202, 81 Stat. 602 (Dec. 15, 1967), codified as Chapter 14 of Title 29 of the United States Code, 29 U.S.C. § 621 through 29 U.S.C. § 634 (ADEA), prohibits employment discrimination against persons 40 years of age or older in the United States (see 29 U.S.C. § 631(a)). It also applied to standards for pensions and benefits provided by employers and requires that information about the needs of older workers be provided to the general public.
The ADEA was later amended in 1986 and again in 1991 by the Older Workers Benefit Protection Act (Pub. L. 101-433) and the Civil Rights Act of 1991 (P.L. 102-166).
The ADEA differs from the Civil Rights Act in that the ADEA applies to employers of 20 or more employees (see 29 U.S.C. § 630(b)) rather than 15 or more employees. Both acts do, however, only apply to employers in industries affecting interstate commerce. The 20 employees can include overseas employees Morelli v. Cedel (2nd Cir. 1998) 141 F3d 39, 45.
The ADEA protects US citizens working for US employers operating abroad except where it would violate the laws of that country - ADEA 29 USC §§623(f)(1), per Mahoney v. RFE/RL, Inc (DC Cir. 1994) 47 F3d 447, 449.
An age limit may be legally specified in the circumstance where age has been shown to be a "bona fide occupational qualifications reasonably necessary to the normal operation of the particular business" (BFOQ) (see 29 U.S.C. § 623(f)(1)). In practice, BFOQs for age are limited to the obvious (hiring a young actor to play a young character in a movie) or when public safety is at stake (for example, in the case of age limits for pilots and bus drivers).
The ADEA does not stop an employer from favoring an older employee over a younger one, even when the younger one is over 40 years old. General Dynamics Land Sys., Inc. v. Cline, 540 U.S. 581 (2004).
The United States Supreme Court in Meacham v. Knolls Atomic Power Lab, 554 U.S. 84 (2008), held that the employer, not the employee, bears the burden of proving that a layoff or other action that hurts older workers more than others was based not on age but on some other “reasonable factor.” 
The 2008 U.S. Supreme Court ruling Gomez-Perez v. Potter allowed federal workers who experience retaliation as a result of reporting age discrimination under the law to sue for damages.
In Kimel v. Florida Bd. of Regents, 528 U.S. 62 (2000), the U.S. Supreme Court held that state employees cannot sue states for monetary damages under the ADEA in federal court. The EEOC may still enforce the ADEA against states, and state employees may still sue state officials for declaratory and injunctive relief.
ADEA remedies include reinstatement and back pay for employee or damages if reinstatement is not feasible and/or employer's violation is intentional.
Section 623 of the Age Discrimination in Employment Act discusses the defenses to ADEA claims as follows: