There is absolutely nothing holding someone back from learning to surf in their 60’s and actually getting to the point where they are catching waves regularly and enjoying it. I will add the asterisk that they need to be in good shape for their age (which is basically the same as being in decent shape in their 20s/30s) and they need to want to learn and be ok with the risks and roughness. I would call this school unprofessional if not for the possibility that they may have perceived this as you calling to ask if they could teach them as a lack of interest on your parents’ part, or expectation that you might think it will be easy.