16 Years ago, I became certified as a Senior Personal Trainer with the Senior Fitness Association http://www.seniorfitness.net/sfaspt.htm. What I like about the organization is that this is their entire focus and not just one of many.
The drawback for me was that – at the time – they seemed to go through an organizational change, and their recertification process was problematic. At the time, they required that you get the CeCs only through them, and then did not provide adequate material.
I believe that this has changed, and I am toying with the thought of getting this certification again because they provided really good information.