This happened when I was 15
I used to go visit my best friend at his department, their parents just split up so he was alone for several hours at a time because of his mother´s work, so we hanged up playing videogames and reading comics.... you get the picture, right?
Well in one of these visits he tell me that he has a cd with porn that her sister´s boyfriend had given to him a few months back, so I asked him to put it in to watch it, nothing major for today´s standard, just several short clips downloaded from internet and such.
A couple of minutes (and videos) later I have this big boner in my pants (I´m 6.9 inches now , but back then I was like 6.4 or so) and I´m not very tall so it was very noticeable, he sees this and begins to joke around if I needed to go to the bathroom for 'relief' or something, I told him that 'who knows maybe I´ll go' and then he says 'well, in that case I dare you to go!!'
Taking his challenge I go to the bathroom, but he insists in coming with me so I don´t cheat or anything, once inside I pull down my zipper and began the good old stroking on myself, He is like 'dude you are large (I wasn´t aware of it before, but it was a really good morale booster back then)' and he then opened his zipper too and began to masturbate right next to me, he was like 5.8 or so, after a couple of minutes he comments on my technique like: 'why don´t you try pulling slowly the skin back to the shaft and then let go (Both of us are uncircumcised)' and then he says ' may I?, like this' and he grabs my dick and begins stroking it very slowly, quite an unusual feeling and I´m like 'wow it feels good, but have you tried doing this?' and I grab his dick and began to make a rosy palm with it (pulling back the foreskin and rubing his glands with the palm of my hand) and he´s like, 'dude it hurt a little but don´t stop now', and I say 'don´t you stop either', we keep going five more minutes and he told me that he was about to cum, so I was and we changed to the fist and kept masturbating each other to orgasm and we aimed our penises so we ejaculate in the toilet, I came first and he did a few seconds later.
He looks at me and says 'that was cool, I was seriously thinking you would chicken out, can we do it some other time again?'
Unfortunately he moved away a couple of months later so we didn´t get a chance to repeat it again, a true pity for I was eager to repeat it...