Thank you for your question today, I look forward to assisting you. I bring nearly 20 years of legal experience in various disciplines.
If you believe that your age is the basis for this treatment, then the best thing that you can do is say that to them tomorrow, outright.
I know that many people feel that that is a dangerous thing to do, but in truth, if that is your belief, then you need to make that clear to the employer. The employer must then take that argument seriously, because to ignore your allegation is to essentially state that the company supports age discrimination.
Now, you don't tell the company so that they'll help, because usually they don't help. They become defensive and say that your position isn't supported by their investigation. You say it to the employer because if you don't, and you wait for them to discipline you or even terminate you, then you give the employer a defense against your claims....the defense that you never gave the employer the chance to cure the issues.
So, bring a pad of paper and write down everything. Tell them that you believe you are being discriminated against based on your age. Give clear examples (like the three false allegations and the other bullying issues). Then see what they do.
That's really all you can do, in advance, to prepare.