When I was a pre-teen I asked my mom about it because I'd been playing The Sims 2 and figured out that your sims can woohoo or they can try for baby, and both look like the same thing. And of course 1 sim day after the lullaby jingle plays upon woohooing the female sim enters the first trimester, and 3 sim days later you have a new baby sim in the family. She was very factual about it all, but didn't bother to talk at all about anything other than explaining vaginal sex. I had to piece the rest together from what I learned at school.

Fortunately I had fairly decent sex ed, except it was painfully boring and felt no different talking about human reproduction than when in highschool biology we talked about how plants reproduce (complete with extremely vague heavily photocopier-burned diagrams of anatomy that look almost entirely unlike what it's depicting which we had to label) but they at least discussed condoms and birth control pills, and even demonstrated a condom on a wooden phalis when I was in high school so that's a lot more than I'm sure some kids get

For my own kids, my oldest is 5 and has already asked. I've left it extremely scientific because she's way too young for a proper talk in just explaining that a male secretes sperm that fertilizes an egg which eventually forms a baby. She wanted more detail but I had to leave that at "when you're older". I'll probably have to give an updated talk when she's 7 or 8 to make sure she knows about periods and maybe I'll then go into more detail so she can be armed with knowledge should any boys take an interest in her (and statistically many boys have watched porn by age 10 which is terrifying)