In our family nearly all the women on both side of the family started quite late 14/15ish. I was 14 and very well developed in other ways. My 14 year old hasn't started hers yet but she is autistic and takes a lot of medication so I don't know if this would affect it. My 12 year old (who is away from home poor thing - typical) called me this morning to say she thinks she's started hers. I was a little surprised because she's tiny - nowhere near 7 stone and very petite. Although she does have underarm hair and seems to have grown up a lot in the last year.
I just assumed her sister would start before she did - it's obviously not that predictable I suppose.