I remember first officially learning about it in school in 5th grade.
That seems like an okay age. Dudes are beginning to think girls aren't as icky anymore and some girls are starting to get their periods.
Many girls these days are hitting puberty as early as the age of seven, mostly in African American girls, judging from a study I read up on. Other girls hit it nowadays at around eight or nine, perhaps? For boys, I'd imagine around nine to ten, maybe.
Soo I'd say fourth or fifth grade. Perhaps even third grade by the way things are moving so quickly.