Employment Law Questions? Ask an Employment Lawyer.
Have you voiced to HR that you believe the basis for this decision was your age and do you have evidence, other than the fact that you were replaced with younger people, which suggests your age was the factor here?
That's really the issue. In employment law, unless you signed a contract that expressly stated that your terms of employment could not be altered without your consent, they can be altered at any time. This is called "at will" employment. So, the only way for this to be illegal is if the only reason you were moved was because this person was discriminating against you simply because you were older.
If the manager can give any other reason (numbers issues, change in product, personal issues between you and him, the younger people were also people that he'd worked with before, etc.) that is legitimate and non-discriminatory to age, the decision would be legal.
If you want this to be investigated because you believe age is the motivation, you need to file an EEOC complaint and have them investigate the matter for you. It is free and it is a protected complaint, meaning you can't be retaliated against for filing it.