Unfortunately I started puberty at 7 and NOBODY explained anything to me until I was over 13 and it was way too late for anything because it had already happened, I'd already done it and I'd already figured it out for myself.
I learned everything for myself with the help of curious friends and the internet.
I don't think Kindergarten is too young as long as the content and presentation is age appropriate. Ex: boys and girls have different parts, etc. Though it doesn't need to be too much more in depth at that age. I think sex education should start young with age appropriate content and presentation and then continue with gradually progressive lessons until said kid turns 18.
Too many kids don't get appropriate or acceptable sex education at home so I think it's important that they're given the opportunity to learn about it in a non-biased environment where they won't be judged for having questions.