Very interesting post.
Though I wonder if it happened quicker than usual for you - and that due, in part, to a dire year at about that time of life. I'm 36, and am slowly finding my memory is not managing everything. I'm sure it's partly because I've more projects/things on the go than I used to, but...
Now what was I going to say? Heck, forgotten, never mind :)
I'm sure another lesson is that life quality (health, sleep, happiness) affects our brain performance, big-time. Also, if I'm not interested, then my IQ does a bit of a downward slide. Reversible, I hope.