I'm 16 years old, and have been with my girlfriend for a year, I've never had this problem however recently (past month or so) I've not been able to keep it up during sex, it's usually just before she's about to cum and it goes completely flaccid, ruining the experience for us both. I'm not taking any medication, I eat well and I'm a healthy weight (140lb 5'10"). Has anyone else experienced this at a young age- if so, do you do anything to combat it?
Just to clarify, 16 is the legal age where I live.