I think everyone, including the OP when she calmed down a bit appreciated that it was reasonable of the woman/women to query the boys age. However there have been some posters who wouldn't be happy with him being there regardless especially as he "looks older"
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The OPs anger was misplaced but who of us hasn't vented to or at the wrong person sometimes? The woman/women in this case were the trigger that pushed the OP over the edge.
Who knows if these women would be any more comfortable knowing he was younger, or just accepting because he wasn't breaking the rules? The point attempting to be made was that if a young boy in the changing facilities made you uncomfortable you could use a cubicle rather than make life more difficult for a parent and child with additional needs. Or you could open your mouth and speak to the mother and establish where a compromise is most easily made rather than leaving her feeling that someone has gone behind her back.
Some people prefer to change in the open but the reality is that they could compromise and use a cubicle. I'm sure OP would love to be able to do the same but that isn't possible for her situation.
And I agree there isn't a magic age that one day it is fine and the next day it isn't. However, the staff can't be making every judgement on a case by case basis so they set a standard which in this case is under 9 (so in the example given above, in fact a 9 year old on their birthday actually isn't within the rules anymore). What age the child looks is completely irrelevant.
Women and girls have exactly the same recourse as Mmn and boys do in the alternate scenario in that they can choose to go elsewhere, go at a time where children are unlikely to be there, they can use a cubicle or they can campaign for change either to support joint changing facilities incorporating sufficient family and additional need changing, lowering the age limit or whatever they feel is appropriate or simply mump and moan and make parents who already have a difficult job feel worse.