In stages, as needed.

Build an environment of trust. Let them take the lead. Pay attention and there will be opportune times to have all of those things— I.e., “teachable moments”. Honestly though, it’s not a one shot deal. It starts off when they are just wee ones and you use the correct anatomical terms for their private parts.

Whenever they want to learn about it. If children want to learn about sex, they should be allowed to. If they don't want to learn about sex, they shouldn't be forced to. It should be up to them to decide. There's no need for sexual things to be taboo.

All humans belong in isolation pods with data transferred to their brains and come out at age 20

Whenever they're curious about it - don't make it awkward and shove it down their throats, but don't make it taboo either

as soon as it becomes vaguely relevant (they start talking/asking about it, they see something on the news, their friends mention something, an adult in the vicinity says something relating to sex). if you "protect" children from knowing about sex, you are doing them a disservice. some people have no idea how much groping, sexual abuse, sexual assault kids endure when they're younger but the kid has no clue what's happening because no one taught them what constitutes appropriate touching vs. what is non-consensual & sexual touching.

if they have a chance to do it... if not probably it's better they don't ever know of its existence

Not as children. Just talking about loving each other when we mean having sex. Nothing about an errect penis that needs to penetrate the vagina etc.

what Kate said

As soon as they start asking about it in any form.

When they hit puberty. I wouldn't make the focus on it be about sex but more about the changes they'll see and feel.

Idk these days, they be knowing what that shit is from 8+ so from 10/11. Bc from 10 onwards is when kids start fully getting to grips with what's okay and what's not as okay as they start gearing up to enter the next chapter of their lives before hitting puberty and shit