I probably started around 4th grade.
It was after I started getting my eyebrows waxed (and that was around 8)
I believe my older DD (now in HS) started around the same age (9-10)
My younger DD just turned 9 (going into 4th grade). Last spring/summer she said her leg hair bothered her and asked if there was something we could do about it. I shaved her legs for her, probably 2-3 times over the course of the summer. (So she was just turning 8)
She just mentioned it again a few weeks ago, so I helped her shave them and taught her how to do it.
I am very hairy, milky white skin, black hair, and I have hormonal issues. I grew up in FL and did not own a pair of shorts and almost always wore long sleeves or a sweater.

Every day I think back with regret about the fact that I was a thin, attractive teen but I felt I had to hide my body at all times and had horrible self-esteem that led to other problems.
My older DD started getting teased in early elementary school. Over the last few years we have dealt with bulimia, self-mutilation, and other issues. If my child feels more self-confident and receives less teasing without body hair, I'm going to let them shave.
I don't want her to start too early, because once you begin you can never stop.
Just curious what you mean by this?
It's the same hair regardless of how often (if ever) you shave it.
I know several adult women who chose to stop shaving their legs.