I started writing ideas in blue books when I was very young (like 7 or 8). Most were stupid, but a few were good at the time, and in fact later become commercially available products. Of course, I always thought "Hey! I thought of that first, but I'm eight and can't do anything about it".

I only did this sporadically (regrettably - not only would I have a lot of ideas by now, but I'd have something humorous to look back at and/or give my kids), but I started having lots more ideas toward the end of my undergrad time and during grad school. My idea generation dipped for a while after grad school because, you know, life gets in the way. But it has picked back up again as my life has stabilized and I've started keeping a tiddlywiki of ideas. Most bad, a few good. None brilliant.

First non-trivial idea? I don't remember. I think it may have had something to do with a novel way of feeding cows or something like that (I grew up on a semi-working farm). As if cows need a new way to eat.