I honestly cannot remember my mother sitting in with me during an exam except for once, when I was around 16 or so and the school med exam indicated that I had too high a heart rate and was referred to the family doctor. My mother acted like she was at my wake, as if getting a death sentence from the doctor was just a matter of time.
I suppose she must have been with me when much younger, but I cannot really remember. I do know though that, however justified her concerns about my health, she always acted like the doctor was going to announce I had a terminal disease, like I was one step away from the grave. It was maddening and I hated it beyond words. It made me feel like an utter wimp. A good rule of thumb is that boys do not like to be made to feel like wimps or sissies.
So yes, I would say that at some point boys are too old to be with their mothers during exams. If chaperones are required to be present, far far better to have a stranger or neutral person sit in, even a neighbor as far as I was concerned, but not a family member who you'd have to see every day at home and could not escape their emotions and concerns. Mothers are just too emotional and fathers too distant and disapproving and overbearing.
Far better to have an unknown, neutral but trustworthy person. Besides, in the long run, boys are not going to have their mother at their side for long in life. Better to get used to standing on their own feet or get used to other people in uncomfortable situations. That is what the rest of life is going to be like after childhood.