I thought about this when I was in middle school and never really got an answer. Considering her age, I'm assuming that she started puberty during her stay in the attic. However, we all know that there was basically no privacy in that tiny spice. Also, the beginning of a girl's menstruation cycle is monumental for young females. Was this just not the case? It was never mentioned in the diary.
Some quotes from her diary -- she definitely wrote about it. From The Diary of a Young Girl: The Definite Edition, Translated by Susan Massotty, Edited by Otto H. Frank & Mirjam Pressler. Bantam Books.
P.S. I forgot to mention the important news that I'm probably going to get my period soon. I can tell because I keep finding a whitish smear in my panties, and mother predicted it would start soon. I can hardly wait. It's such a momentous event. Too bad I can't use sanitary napkins, but you can't get them anymore, and Mama's tampons can only be used by women who've had a baby. (Monday, November 2, 1942)
I think that what's happening to me is so wonderful, and I don't just mean the changes taking place on the outside of my body, but also on the inside. I never discuss myself or any of these things with the others, which is why I have to talk about them to myself. Whenever I get my period (and that's only been three times), I have the feeling that in spite of all the pain, discomfort and mess, I'm carrying around a sweet secret. So even though it's a nuisance, in a certain way I'm always looking forward to the time when I'll feel that secret inside me once again. (Thursday, January 6, 1944)
I hadn't had my period for more than two months, but it finally started last Sunday. Despite the mess and the bother, I'm glad it hasn't deserted me. (Wednesday, May 3, 1944)