I started at 10, the earliest of my sisters or my mother (my sisters, however, developed much more slowly than I did...by 11 or 12 I was a DD and being mistaken for my 16 yo sister).
I did a huge report during my college years about this, and there is a lot of controversy. I wasn't overweight, but I was curvy and needed a bra at 10. My sisters, however, started their cycles around age 15 or so, and were not (and still are not) as curvy as me. I'm not obese, never have been, nor am I of the athletic/aerobic build (I'm more of a weightlifter than a runner).
My daughters are 14.5 and almost 12, and still haven't started. However, for the past 7 years I've been either pregnant or nursing, and they are homeschooled, so I do think that has a lot to do with it. My eldest is a swimmer, too, and super muscular, my almost-12yo is athletic but is catching up with her sister in the development dept. I fully expect that within a few months both girls will begin cycling with me (my baby is 20 mos old and my cycles are fully back).
My step dd is 15.5 and started years ago, but she grew up drinking milk and eating a ton of meat, whereas I didn't let my girls drink cow's milk until about a year ago, and even then they rarely want it. StepDd has a totally different body type and is not nearly as physically active as the other girls, so that is also (I believe) a factor.
I'm glad my girls didn't start at 10 like I did. Having DD breasts at 11 was incredibly difficult socially, and my cycles were so heavy I was constantly ruining pants at school, even with lots of protection. I don't know how many days I spent with my jacket tied around my waist, which is like a neon sign saying, "I just bled all over these jeans, thank-you-very-much. Feel free to point and laugh."
Hopefully, when my girls do start, they'll be ready and will have lighter cycles. I know swimmers, especially, are more likely to wait longer to cycle at all and to have lighter cycles (she swims 5-6 nights/week for 1.5 hours).