Honestly, I don't believe much in all that censoring we do with the kids (unless we're talking about R rated stuff, or stuff that is really explicit, whether violence or sexual stuff). But Buffy, I think you can pretty much watch that show at any age as long as you watch with an adult that will be able to explain things to the kid. (Also, as long as the kid doesn't get scared of vampire stuff).
I watched the whole show when I was 11-12 on my own (with a few eps with my dad or brother), and I think it was fine. My dad knew I loved horror movies already, and my parents never shielded me from sex either, they just had an open mentality. And heck, I was so addicted that I'd talk to my mom for hours on end about Buffy, so if there was anything wrong well we could talk about it.
On the flip side I had a friend who couldn't watched the show cause the mom saw one scene where Spike was chained up and she was like "nope, you're not watching that". My dad was just like ya sure you can watch, just realize that this shit going on, it's not healthy.
If you're worried about later season which are a bit more graphic, you could always start a kid on s1. I feel like s1 is pretty tame. Than you can just make them wait a year for s2, just like back in the days 😛