I had been prepared for it thanks to previous discussions and a good early sexual education program in early primary school. I was 11 and mum just gave me pads.
The only bad thing was that she told dad. I asked her not to... And I get as an adult why she might tell him anyway but he was a dumb person to tell... Almost immediately he came out to question me about it. I felt I couldn't trust her from then on. (I do now, but it took me a long while after that).