I know at least four people who started college by age 15. They're not "kid" geniuses anymore though -- the youngest is 16 and slowly going through college part-time, while the oldest is in his 30s and a full math professor at Arizona.
I don't know about the upbringing of the other three, but one attended a program where taking classes mutliple grade-levels ahead is the norm (though no-one else learned calculus in 3rd grade), and attended Canada/USA Mathcamp during the summers of his undergrad.
I second the Olympiads. Terry Tao famously represented Australia at the IMO at age 10, so he's definitely old enough.
I'm friends with an incredibly smart kid. He's 14, but has been put up three grades in school at one point. He does all the obvious enrichment things which are available in the relatively small Australian city he lives in.
His life experience has been pretty unusual. He doesn't really know what it's like to be challenged in school. All his friends are way older than he is. (Once, I asked him how being constantly around people older than him made him feel. He replied, "Concerned for my future.")
He doesn't know anyone like him, which I think is a shame: he'd probably get along very well with them.
Does anyone know any similar kid geniuses? If so, can I give them my friend's details?
Thanks.