It is legal to ask. If you feel you are discriminated against because of your age, you should discuss the matter with an attorney However, due to the fact this is a volunteer position, it is hard to picture what relief may be available.
I am licensed in New Mexico and Pennsylvania, and therefore any discussion of issues related to other states must considered within that context. In addition, my comments are not intended to create a legal representation but merely to respond to the limited facts presented by the question. Any opinion herein is not meant as a precise statement of legal rights or as a recommendation of any particular course of action. A more complete legal review can be obtained through local counsel.