tenure in US universities

I am quite clueless about the subject in the title of this post. But from the agonising I read in various blogs, and the experience of some relatives, I conclude that getting tenure at a good university in the US is a difficult thing.
But, I did not know it was this tough-
" it was not until 1987 that he began to teach at Yale, where he earned tenure in 1999." 12 years to get tenure for guess who--------
Benoit B Mandelbrot.