My goddaughter rang me to tell me she'd started- she'd been fretting it'd never happen (she's 12.9).
I did a bit of 'omg, what happened to that little squishy baby you once were', so that she could roll her eyes at me, segued into the fact she'd felt pants today, and finshed with plans to pick her up in the morning as I'm going shopping, and she fancies a couple of hours out.
I think it's enough to acknowledge it - she wanted me to know, I was quite laid back about it, but very touched that she rang to tell me.
I'll pick her up tomorrow morning, we'll potter about for a bit, and finish with cake somewhere, at which point I'll get into a 'blimey, you're almost grown up' type conversation with her.
She'll feel very special, as she's one of three girls so 1-2-1 attention not to be taken for granted. I wouldn't buy her a special present to celebrate, but I will almost certainly treat her tomorrow, because I'm feeling all soft about her - it just won;t be a 'special' present that she'll remember, it'll just be something that's nice tomorrow.