What are the ways a Senior can prove they were denied employment due to age discrimination?

Before taking any action, seniors must consider four factors to prove age discrimination or denial in employment.
1. The senior is among the related age class
2. A younger employee is treated more favourably ( usually under 40)
3. His job performance was satisfactory, and he is also qualified enough for the offered job
4. Unfavourable job action was taken against him
In case of a complaint, the employers always have a pretext. However, the employee must prove that these are all pretexts to deny the discriminatory behaviour.