I really thought I had explained this somewhere here, but apparently not, so I'll do it (again or not, as the case might be).
Here's the dealio: when I was younger (back when dinosaurs ruled the earth) I took the then standard IQ test (Stanford-Binet) and my results were 165. Now, forgetting for a moment that IQ tests only measure what IQ tests measure, by the Terman scales (which are the ones usually used) anything over 145 is considered genius level.
Truth be told, I felt like a genius. I always got all the answers right on tests from elementary school throughout my University days without even studying and *never* paying attention in class (which of course made me incredibly lazy and is the reason I don't get much done even to this day. That's my story, and I'm sticking to it no matter how much my wife argues). I wrote a 200 page single spaced typed novel at the age of 11 that convinced my teachers I was going to be a famous author (the less said about that the better, though). I got a home computer before folks even considered having home computers (I actually got the second one sold in my state) and mastered programming and was soon teaching it at the University despite the fact that I had no credentials at all in the subject (or even teaching, for that matter). I wrote software that, for fifteen minutes at least, had me world famous and convinced I was going to be rich (but that's a whole other depressing story).
But let's stipulate for the present that nearly 4 decades ago I was at genius level. We have all heard that we lose a number of brain cells (neurons) every day -- the number is estimated at anywhere from between 10 thousand and one hundred thousand. It does help that we start off with about 100 billion of the things but, still, when you are losing something on the order of 30 million a year you can see that soon you'll be sitting around on the floor trying to see if you can call your cat by telepathy (note: you can't, no matter how long I've tried).
So for the longest time I've just assumed that sometime around my 50's I had lost enough of my neurons that I was no longer a genius, and started referring to myself that way. I meant it quite seriously (well, as seriously as I mean anything) and although it was a little depressing at least it gave me an identity.
Now, for those of you older people (or even young, nervous types) I should mention that there has been a ton of research on the subject in the last decade or so, and most of the folks in the field of brain studies nowadays believe there is more optimism than first thought. While they have always believed neurons couldn't regenerate, it has been found in monkeys at least that thousands of neurons are born in the cerebral cortex each day. And a study at the close of the century found that a group of brain cancer patients had sprouted new brain cells in the hippocampus. So at least your monkey is going to be just fine as he gets older and, as for you, if you get brain cancer you may just be in luck (why neither of these things is cheering me up I just don't understand -- it must be that I am an ex-genius).