When should kids use public toilets alone?

The question bugged me during a dinner with some family friends a few months ago. The family we were with are frequent visitors to this restaurant. But seeing the girl going inside the toilet all alone, she's only 5 years old at that time, was still a surprise.

I only found out that the girl used the toilet because I had to weave thru the restaurants tables and chairs searching for the toilet because my girls must go, too. I even had to ask the girl what she was doing there all alone, a bit worried myself; but she just shrugged and said she needed to use the toilet. I am amazed and worried, to be truthful.

I wont even think about letting the girls use the public toilets alone. Especially if I am there with them, if I could go with them. My girls dont want to use public toilets alone, even if we urge them to go together. And we only ask now coz MC's almost seven years old, you know, just checking if they would really jump at the chance. Well, they wont.

The first complaint being they wont be able to cover well the toilet seat with toilet paper, as I always taught them to do, before sitting on a public toilet. Meaning, they were concerned more on their hygienic than being afraid to go alone. Something else to think about, huh.

Call me a worrier but at the young age of 5, I think it is still so risky. Or is my judgement colored by the story of a girl who saw a masked man in the public toilet in the public school here?

How about you, when do you think kids could use public toilets alone?