I got my period a month before I turned 16 - in health class they had told us if we didn't menstruate by the time we were 16, we might need to go to the gynecologist and get checked out.
I know girls are reaching puberty earlier and earlier (I blame the chicken - LOL). I would ask your gyno at your next appointment at what age you should be worried about her, I personally think 12 is too early to worry. Also, I don't think bone age and menstruation have anything to do with each other. A lot of girls grow early and then stop growing by 12 or 13. The other signs of puberty, breast development and pubic hair, may be more telling, but I'm not sure they really coincide with an immediate start to menstruation, every body is different. Talk to your gyno or her dr to find out at what age additional tests may be warranted. They may do them right away to make you feel better, hopefully they'll tell you an age (i.e. 16) where they would be concerned.