This happened when I was 17
Even though my next door neighbour Marc was three years younger than me we were always hanging out together. I suppose it was because there were not too many kids of our age living on our street as both our set of parents married late in life.
We used to go swimming every week to the local community pool on Saturday. One Saturday during summer school holidays, shortly after Marc's 14th birthday I was in for a surprise. looking back I had not noticed how much he had grown in that year but today I was about to find out! Normally after our swim we would both take a shower which were communal, me in the nip but he always kept his shorts on. This Saturday we were the only two in the showers, me naked and Marc in his shorts.
As I was showering Marc said 'as we are the only two here I might strip like you' I told him to go ahead as we are just guys taking a shower. As he dropped his shorts I got the shock of my life. Now while I am by no means small(4 soft, 6 hard) Marc's flaccid member hung a good thick 6 inches. My eyes were like saucers and to be honest I was a little jealous that a 14 year old boy could be so well hung.
Marc noticed this and said 'you look surprised Tom', with a sly grin. I replied that he was a very big boy indeed. With that he started to soap it up and said 'if you think it's big now just watch how big I can get it'. With that he got fully erect to about eight solid inches. By this time I was hard myself and I said 'I've got to get rid of this' he agreed and asked if I minded if he joined in. I said sure. We went at it and I only lasted about two minutes, Marc smirked and after about another five minutes blew an unbelievable load. After that day we had many good jack off sessions.