I'm usually on the more protective side of the parenting scale, but I've been letting my son use the bathroom alone for the past year or so. And he just turned seven. I think I started letting him use the restroom alone when he hit school age. So around 5.5 or 6 or so.
Granted...and this is probably a huge granted, we almost never go places where there are large busy restrooms. Usually when he is using a public restroom it's either at church, or maybe a school (for chess tournaments) or the library. Maybe a public park. Very occasionally a restaurant, but we don't go to restaurants often. Shopping is also rare. So are museums/zoos. So, usually if we go someplace, it's someplace that would only have a few stalls. We don't belong to the YMCA or anything like that.
There are certain situations, (maybe a large, busy travel center off an Interstate) where I would not feel comfortable allowing him to use the restroom alone. I take it on a case by case basis, but with most smaller bathrooms, I'm totally fine with it.
Like all parenting decisions...it's a risk/benefit analysis,
Risk: The chance that there is a pedophile lurking in the bathroom, just waiting to molest or kill little boys. I really feel that this is a very, very, very, very, very, very, very, very, very small risk. Very. Small. Yes, it could happen. Or I suppose he could get kidnapped. But that seems even less likely as most bathrooms have only one way in and one way out...and I always stay out by the door. It's also possible that I could get murdered going into the women's bathroom. But, that hasn't stopped me from using public restrooms when nature calls.
Benefit: He learns independence and doesn't have to feel uncomfortable going into the ladies room. No one else is made uncomfortable by him being in there.
The biggest benefit however, is that he gains some independence.
The point of parenting isn't to do everything humanely possible to prevent any potential harm from ever coming to our children. The point is to teach our children to grow into responsible, healthy, capable, functioning, happy adults.
Now, I might feel differently if we went to the YMCA or places with pools where he had to change. In a situation like that, I think I would have him change/shower at home and then go straight to the pool. Actually that is what I have done, the few times we've been in that situation. Still, even in that case, I'm not so worried about strangers, but about him getting lost (those bathrooms are HUGE) or needing help (not being able to turn a shower off or on or it being too hot/cold).
I'm certainly not criticizing anyone who doesn't allow their 6 or 7 year old to use the bathroom by himself. If you're not comfortable with it, that's okay. In many other ways, I'm on the more protective end of things.
Of course, a decision like this also depends on the child. My son is responsible and not going to just fool around or play in there. He's not going to dawdle or daydream or splash water or make a mess or do anything like that. He just goes in and out as quickly as possible.
Taking all that into consideration, I'm okay letting him use the restroom alone. My children tend to err on the more introverted side of things, So, I'm always looking for ways they can be a bit more independent. This is one small way to encourage that.
What about you? At what age do you let children use a restroom alone?