Do you think I'm overreacting?
I mean, it's all subjective. I started watching the show live when I was about 5/6. At that age, a lot of the themes (mostly sexual related) were kinda lost on my at that point. I didn't find anything that terrifying, but again, that's subjective and you'd have to know the person to make that call.
My personal, "Episodes to watch out for":
Smashed - Some of the most graphic sex in the show
Dead things - Lots of sex
Intervention - Buffybot sex with Spike
Where the Wild Things Are - An episode so sexy is rose a supernatural force!
But really with this question, you have to know the person. Some 12/13-year-olds a pretty mature by that point. They've got the whole sex thing figured out and they're socially aware of issues such as rape, sexual abuse/violence, drugs, death/mortality etc.
As I said, for me I missed a lot of the meaning due to my age when the show was live. I mean by the time it finished I was teenager, so it's slightly difference.
My personal opinion is that 12 is fine, as long as the 12-year-old in question is at least of average maturity for their age.
This is such a personal opinion/situation based question, though.
What I might find inappropriate, you may not and vice versa :)