What does, or does not, count as age discrimination is indicated here:
https://www.equalityhumanrights.com/en/advice-and-guidance/age-discrimination
However, as others have said, it's incredibly hard to
prove age discrimination, which even the the Equality and Human Rights Commission seems to accept.
For example, when I was unemployed and job-hunting, I saw a vacancy advertised for a post that almost exactly matched my qualifications and experience. The newspaper ad stated that the employer was looking for a graduate, which was no problem for me as I've got an honours degree in mathematics. However when I sent off for the application package, I noticed that 'graduate' had been replaced by 'recent graduate'. I wrote to EHRC, stating that I felt the employer was using age discrimination because it was far more likely that a person in their twenties would be a 'recent' graduate than someone like me, who was in his fifties.
The EHRC wrote back to say that they fully agreed with me that the employer appeared to be discriminating on the grounds of age but they felt that they would be powerless to act because of the difficulty in proving it.
So even getting the EHRC to agree with you that you're being discriminated against on the grounds of age probably isn't going to help you.
Sorry!