That's cortisone, Cortisol is a hormone

I think you're right! I definitely have a lot of ideas and stuff bursting out of my head at all times. Unfortunately I think most of those ideas are nonsense haha

I think it's less about growing older and not having time, and more about the fact that as you get older, you branch out less and less. You are content about staying with what you "know" best. It's kinda like teaching REALLY old people how to use a computer, only not on the same scale :P. And we do have books on Mars, we just use them as paper weight instead. It's very troublesome that authors have to write a WHOLE book, just so we can use it as paper weight, but it is what it is!
And I'm sure if you find a big enough envelope, you can probably squeeze in a brain in there!

I just never got into it. As a child I was busy playing outside, and as a young teen I got into video games. Movies also became more wide spread, so it was just all around easier to "watch" a book, than read it. I read stuff, just not books ^^.
As for the "wrap my head" saying, it's probably a figurative way of saying you are enveloping your mind around a problem. It works, even if it is a little clunky!