I was 12. I told my mom, and she told everyone and their uncle, much to my embarrassment.
I did not tell my friends at school, because the leader of our little group had not got her period yet. I know she would have called me a liar in front of everyone, because there would have been no way, in her mind, that I, could have gotten my period before her. Getting your period before others was a strange status thing.
I'm 46, and this month my period started on day 17 of my cycle. I am so ready to be done with this. I'd at least like to be missing a month or two at this point, but I guess I'm a few years away from that.