Actually our YMCA is extremely strict about opposite sex in the dressing rooms. I do believe it is 5 and under may be 4, and they mean it. They do have some individual rooms like dressing rooms you can go in with kids but absolutely not into the dressing rooms.
And even tho I lived on a coed floor we had single sex bathrooms. all the dorms at my DD's school have single sex bathrooms.
The big problem with older boys in the women's room is the older girls. What older girl wants to come out of the stall after, sorry about this, passing gas, etc and have a boy her age standing there? How about a girl dealing with her period and a ten yr old boy is standing there gawking at her. Girls at that awkward stage are embarrassed by life enough as it is without having to deal with it in the bathroom because of over protective Moms.
What if you spilled something on your shirt and would like to lift it up to clean it, can't with an 11 yr old staring.
I always say if your son needs to come into the Womens room take him in your stall with you. If he is too old to be in there with you then he is too old to be in there with my daughter or me.
But remember that the moms taking their older sons in the bathroom feel their right to protect their sons outweigh our rights for our daughters to not be made uncomfortable. My girls do not like seeing boys their age and older in the bathroom.
Shoot we were at Walmart last weekend when a mom and her kids were coming out of the bathroom. Imagine the look on the 2 -12 year olds and the 8 year olds face when they saw a boy that went to school with the one 12 year old coming out of the women's bathroom. The older girls started laughing after the initial shopck wore off because they did not know how else to react. The child is a normal 7th grader at my 12 year old's friend's school. I was just shocked.