You're right, people are different which is why for someone to put an age on when boys should not be in a women's room will never be agreed upon. Personally, I don't see what the big deal is one way or the other. I go in, I pee, I leave. Whatever I am doing, it is behind closed doors. I don't like kids peeking in, but that applies to girls too.
I see it as a pretty big deal. Not when the kid in question is six - although that is starting to push it - but eventually, you have to let go. What is a good age? When do you trust them to be able to handle themselves.
At some point your child will be an adult, and that adult will need to use the restroom in a truck stop. Or need to travel - alone - to a different country on business. Your child will need to be equipped to watch for danger signs going to a parked car. They'll need to be able to go off to college, or basic training - join the peace corp. Be equipped to raise their own kids. That process starts the day you bring them home from the hospital, and you don't have as much time as you think you do to raise independent kids.
There are always bad people out there - they don't stop existing when your kid is ten or fourteen or thirty four. You need to equip your kids to deal with it. A Disney bathroom is probably one of the safest places to start teaching it. And a whistle is a great idea.
And if moms of boys going to bathrooms are this scared, its amazing that those of us with girls (I have both) ever let them leave the house. Because for them, the danger only increases when they are twelve. And its truly frightening when they are eighteen to twenty one and it isn't true strangers they have to be careful of.
We could drop the whole issue by just making all restrooms coed and everyone does their business behind a stall door. I've been in bars like that, and have discovered I don't care, so no one else should either. However, we haven't gotten to a world with coed bathrooms, so there is some point at which its inappropriate to bring a boy into the women's room.