Ooh, age discrimination! Hey, if there isn't a legal reason (some places that have machinery you HAVE to be 18 by law) why not ask the director or whoever is highest up in the organization if you got references if you could volunteer. They may be shying away from younger people because they are afraid they'll be unreliable, or because they don't want to cut into students' homework time - if either of these are thier reasons, perhaps you could prove that it wouldn't be the case with you, and be the exception!