,topsyturvey of course it's not ageist. You have reasonable concerns based on your experience of your mother's driving.
Why not tell her she is dangerous on the roads and must stop driving? Then, when she refuses, put the onus on her to prove that she is safe, by consulting optician, doctor and DVLA.
If she is fine to drive, it wouldn't matter what your motives were for wanting to accompany her to an optician's appointment, as she would come out of it with flying colours anyway.
How do you know her doctor or optician have said nothing to her? Would she tell you if they had?
Think hard about a possible injury, or death, she could cause then take action.