The quick answer is never.
Why in the world do we burden ourselves with this chore? Who was the woman-hater that came up with this? And why did we all buy into it?
One of my very first posts on my former, beloved, and now defuncted blog, Hot Fruita Moms, was about shaving my legs or the lack there of. I did a little experiment on my husband to see just show long I could go without shaving my legs before he balked. If memory serves, it was a really long time. I had about a week and a half worth of growth and let me add, I am a hairy woman of fine, strong, German stock! I waited and waited for him to notice and finally I propped my leg up on the dashboard on the way home from a soccer game and he was like, "Hey! Whoa! Look at that!"
He didn't love me any less, he didn't gross out, he didn't asked me to shave, he just made note that my legs once soft and smooth were now proudly showing their German heritage!
Fast forward a few years, I now how have 13-year-old daughter who has yet to shave her legs and it doesn't seem to show any interesting in starting. I'll say it, she is my daughter, a regular Mini Me, a chip off the old German block. Part of me, the culturally indoctrinated part, thinks she should shave. The more enlightened side, couldn't be more proud that she is going au-naturel. More girl power to ya, my darling hairy daughter! Stay strong, meine tochter!
When should girls start shaving their legs? Whenever they feel like it.