For me, it was when I was around 16 or 17 I can't remember 
But it was during summer break and a bunch of friends and I went to my then best friends house to chill and have a big slumber party. Mainly because she had a pool. 
That day was amazing, we had the whole house to ourselves that day. There were about 2 guys that were our age and four girls including me, however, my best friends brother was there too and mind you he's four years older than I am. 
We'll call him Pete. 
Pete was very very into video games and was also very skinny, and he's had only two girlfriends his entire life. (At least that I've known of and I've known his sister since I was like 10) Though he was very very attractive.
Anyways, Pete chills with us the whole day, watching movies, eating snacks and getting in the pool. Eventually, the eight of us had just passed out around the living room of my best friend, and long behold. Pete is laying right next to me. I couldn't sleep so I was playing on my phone. Casually looking at some memes and texting people on Kik. I guess I had my brightness to high cause I eventually woke Pete up. Embarrassed I apologized profusely, but couldn't help but notice the tent he was pitching in his swim trunks. 
Eventually, one thing led to another and before I knew it, we were upstairs in his room fully undressed and just going at it. Honestly one of the best experiences of my life. I would totally do it again, though part of me regrets it since my friend was extremely upset I can't control that. :/