Howdy K,

I'm in somewhat of a similar situation, my elderly mother is mentally ill, was all through my growing up years, don't know if she was ever formally assisted or treated with this.
In my own experience, what has helped me along with researching and reviewing my childhood in light of new information and understanding, is to first be in good therapy for myself. I've found good therapists who help me think through what all I observed and experienced, who help me think through what I want to do concerning my mother, if anything. I learned techniques such as how to be assertive, how to sort out my motivations, how to prepare for possible unpleasant results.
67 definitely isn't too late, but I would suggest first pulling in some good counsel and therapy and assistance for yourself. I suppose I see this as holding true for any sort of intervention with another person, whether it be around mental illness or addiction of whatever, first the folks immediately involved need some help and training.

Good luck,
Sarah