Employment Law Questions? Ask an Employment Lawyer.
Your friend is correct that there could certainly be a claim against the employer under the ADEA (if the employer has at least 20 employees) or your state's statute, which has no minimum number of employees to be applicable.
I think the one concern you'd have here though is that you also moved to part time by your own choice. If the other individuals are not part time, then the employer can allege a "legitimate, non-discriminatory basis" for the differing treatment. However, if there are other part time individuals in that same job that are not experiencing the same treatment, then you're claim would be very strong.
It would be worth your time to make a complaint to your HR department, alleging what you believe to be age discrimination and then force them to either correct the behavior or put down on paper what their reason is for the treatment. That helps to set up the next step, which is to complain to your state level human rights commission or the EEOC (if they have at least 20 employees) to have the matter investigated by the outside agency, thereby preserving your right to sue.