I developed breasts starting late age 11, but didn't have my period till I was 16. So maybe I'm odd.
Sis started much younger, before me even. We grew up on a farm, so we had pretty early knowledge of the birds and the bees. Both of us loved animals, so with her she explained how this would let her make babies later in life, using goats as an example. She also explained that while it's kind of uncomfortable, it's ok.
Kids pick up on the mood of the parent, so if you are freaking about her starting, she'll freak too. Just treat it like any other thing, like she grew an inch or something.
Oh, and since I was 16, mum just tossed a box of tampons in my direction when I told her I needed one. Very matter of fact, update the shopping list and move on.
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Every time you think you've made it idiot proof, someone comes along and invents a better idiot.
?the end of our exploring, will be to arrive where we started, and know the place for the first time. - T.S. Eliot