DD started mowing last year when she was 13. I discussed/demonstated safe & proper use, took several swipes across our rather small city lawn, then watched while she did the rest. A couple of times, I had to make suggestions on how to handle the somewhat sloped parts, but she was fine. I watched her mow the next few times to make sure she could handle it and was observing safety rules, and this year, she does it completely on her own.
I should add that she is average-sized for her age, not petite. The only problem she has ever had was once this summer when she let go of the thin bar that keeps it running, and it pinched her finger. Just a question of her finger being a little bit in the wrong place at the wrong time, but not a serious injury or issue.