How can one collect evidence to prove ageism in the workplace?

The only evidence you can collect is by documentation of conversations and incidences. Keep a record and write the names of anyone involved, like the associate or manager who harassed you by what they said or did. Write how the discrimination affected you, for example, the emotional effect of being harassed or missing out on a promotion. Look for proof on social media if you use it to communicate with people from work. For instance, a threatening or insulting message on Facebook could be proof of discrimination. Print out the notes if possible, and take a picture or screenshot. When you’ve determined your claims and evidence, you can complain to your employer about it, either informally or by submitting a grievance or complain to a tribunal. Keep receipts for extra expenses, like interview costs if you were dismissed and had been looking for a job. Ask for help; depending on how much you trust your co-workers, you could ask them if they have noticed it and include their name in your documentation.