OP well I suppose you always know as soon as you are old enough to know about biological sex and/or see your own privates and understand what they are and how they differ from the other sex.
but if you want a point after that, periods. Which I started - incredibly distressingly - at 9. I would have taken ANY drug on offer to stop that - happily I went on the Pill at 17 which made them less awful.
as an adult, the time I really feel it is when someone questions "Ms" whether in real life or online. Men never ever face a battle to declare, or not, their marital status.
I suppose you could argue every time I get my "period" and my Pill script now as well. Or buy a bra. Fucking hell they are expensive.
oh and going to the "men's" toiletries section where I often find the same shower gel lurking, but in a man coloured bottle - and cheaper.