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

If you want to prove age discrimination in the workplace, you’d need at least the following:
• Show you have the same degree of technical ability/experience as your colleague (with clearly described relevant examples)
• Discover other aspects of the job where your skills and experience make you a better alternative.
• Show your age was at least one of the grounds for your rejection and that this was discriminating.

To accomplish the first two points mentioned above, you should learn about your colleague’s relevant job experience and credentials. Be careful not to touch the confidential information you shouldn’t access. Compared to your colleague’s session, it’s also a good idea to ask for specific feedback on your interview.
To prove and provide evidence that your rejection was because of your age, you can also:

• Submit a subject access request
A subject access request would compel your employer to give all documents/records related to you in any form. The purpose of filing is to uncover some age discriminating communications between the managers who received your application.

• Make use of a discrimination questionnaire.
It’ll allow you to ask your employer questions concerning possible age discrimination towards you.

• Obtain witness evidence
You may be able to get witness evidence from someone who has seen written sources, remember overhearing the talks regarding your application or previous incidents of age discrimination toward you, and is ready to offer a written statement of what they have witnessed. Collecting documented witness evidence is, unfortunately, easier said than done. That’s because most witnesses don’t want to confirm what they’ve seen, mainly due to the fear of being victimized by employers.