Age discrimination is the hardest complaint to prove and the most prevalent in EEOC filings. It is hard to prove because the EEOC allows certain actions by the employer due to a sound business reason. It is true that when an employer is attempting to cut costs the easiest way to do that is to terminate the employees who make the most money.
The employees who make the most money are most often those who are over 40 years old since they are often employed at a higher salary level. Courts would consider whether or not the termination was due to a business decision or due to the fact that the employee was over the age of 40 when determining whether age discrimination occurred.
Generally, to show discrimination, a person would need to show that other employees who were in the same or similar situation (such as same job duties or same title) and who were under 40 years old were not terminated or did not suffer the same action. Courts like to see something tangible that is also an indicator of an age bias, such as memos about age or a history of the employer terminating older workers in the past. However, age discrimination cases have been won without any additional paper evidence and with only the testimony of the employee who was discriminated against.