Radioactive atoms, in the aggregate, don't have a half-life. If you measure the nuclear radiation coming from a heterogenous glob of radioactive material, and it takes N hours to drop to 50% of that level, then it will take more than N hours to drop to 25% of the original level. The original material had both shorter-lived substances with a half-life of less than N hours and longer-lived substances with a half-life of more than N hours (and possibly non-radioactive elements as well), and since the former materials decay faster than the latter, the ratio between them falls until the latter materials dominate.

I suspect the same process holds true for facts.

Facts, in the aggregate, have half-lives: we can measure the amount of time for half of a subject's knowledge to be overturned.

The tricky part there is figuring out which half.

Also, this book was a horrible agglomeration of irrelevant and un-analyzed factoids. If you've already read any two Malcolm Gladwell books or Freakonomics, It'd be considerably more educational to skip this book and just read the cards in a Trivial Pursuit box.