ACE has been great, I was originally certified in 1988 as group fitness instructor, later as a personal trainer also with ACE. I agree that the field, though competitive, always has room for excellent professionals. Regarding mistakes, it is paramount to train with the absolute safety of your clients in mind. It is crucial to know and follow the ACSM guidelines. With older adults, mistakes may be more costly to those you train. Getting older brings all the effects of aging and recovery takes longer. Injuries also take longer to heal, and may remain for the remainder of life. Additionally, you will be dealing with co-morbidities that your client may not be aware of. I strongly recommend Spirduso et.al. “Physical Dimensions of Aging”, an excellent and comprehensive text, http://www.humankinetics.com/products/all-products/physical-dimensions-o… . It is worth every penny. Please feel free to contact me if you have further questions or concerns. I would be delighted to assist with your mentoring. This is a growing market that is underserved in MANY ways!