Nobody taught me directly, but I did have the benefit of a school pamphlet. I was eleven, had just sprouted some pubic hair, and between what the pamphlet told me and trial and error, I figured it out, although I was temporarily disturbed by what felt like blood in the darkness of my bedroom. (It was semen, of course.) I was instantly hooked.
Unfortunately, it wasn't something to talk about or admit to doing. I remember being at school at 12 in a group of boys, and one of them was asking each boy, "can you come?" as a pun, like he was asking if you could come to a party, but also if you could ejaculate. Of course, the truthful answer for most of us was yes, but saying so would be admitting to masturbating, which we were almost certainly all doing pretty much every day. I pretended not to know what he was talking about.
So I was self taught, but I did sort of teach my brother. I think he knew what it was, but hadn't really tried it. One summer day, when he was 12 and I was almost 15, we were home alone, lying on my queen sized bed, looking at a swimsuit magazine, shirts off because of the heat. We both had obvious boners through our basketball shorts. I had my arm behind my head, mostly to keep my hands away from my dick, and he suddenly looked over and said, "hey, you've got hair under your arms." "Yeah, so?" "I hope I do soon." "I was about your age when I got mine. You've got pubes, right? And your dick looks pretty big judging from that boner." He pulled down his shorts and underwear, revealing his penis, which had some hair around the base. I pulled down my shorts, too, so we could compare. I reached down to adjust my erection, giving it a little stroke. I was so horny from looking at the magazine and from the discussion that I couldn't help myself, and started stroking lightly.
"Are you jacking off?" he asked. He put his hand around his cock and copied my motion. "Yeah," I admitted. Suddenly, even though I wasn't intending to, I passed the point of no return and came, squirting several ropes of cum onto my belly. My brother's eyes got big. "You can do that?" "You probably can, too."
He stroked faster, then his breath quickened, and he suddenly stiffened and shot out his first cum. "Wow!" he sighed, catching his breath. I handed him some tissues from the nightstand. Just then we heard our parents pull into the driveway, so we pulled up our pants and started playing video games.
That evening, I heard a knock on my door, and my brother walked in. "I'm going to bed, but I was wondering if I could, uh, borrow that magazine we were looking at earlier." I grinned at him. "So you wanna try it again by yourself?" He blushed and turned away. I handed him the magazine and told him not to get any cum on it. He smiled big and ran off to his room.