I see it differently.
The trouble is that wherever you draw the line, some people are going to think there are exceptional circumstances.
If you say there should be some leeway up until the age of 10, why not 11 or 12?
I very much doubt it was a one off, because the reality is that there are many people who think the rules do not apply to them because they have a baby/toddler/autistic child/small bladder/whatever and so you are more often than not likely to find people breaking the rules who either believe they are the only one breaking the rules so it doesn't matter, or just don't care.
Although this is not a trans thread, I think it's worth noting that it is very very difficult in the current climate for a woman, much less a teenage girl, to say, "This space is for women only and you shouldn't be in here."
When that teenage girl plucked up the courage to say it was a female only space, the OP explained that she believed she and her autistic son were more important. That was a teenage girl saying, "hi, these are my boundaries" and the response was, "your boundaries aren't important". That will make it even more difficult for her to speak up the next time.
She didn't want to get changed in front of a nine year old boy but she was basically told that she had to put up with it or wait. And what would be the point in waiting when two minutes later another mother might come in with her nine year old boy, also believing that the rules do not apply to them?
This is why your rules need to be rules, not guidelines, and your cut off point needs to be interpreted strictly, not loosely.