I know there are a million jokes for that question, however this really is a serious question. I had no idea this is the way it worked.
During puberty, the penis will grow in both length and girth. Once puberty ends, there is unlikely to be any further penile growth. That said, some males may experience growth into their early 20s.
Here's the link for more. They go into a lot more detail. It's interesting and totally SFW.
Follow Up On a Veteran's Penis Transplant
I've posted about penis transplants before, but I don't think I ever linked to a follow up on what happens after the penis transplant, especially one that involves serious medical issues. It's about a guy in the military who suffered severe damage to his penis, scrotum, and his mid-section. While he was in Afghanistan he and his team came across a Taliban ambush. They go into a lot of back story, in detail, about what actually happened to him.
Functionally speaking, Ray can now urinate standing up. And while he’ll remain unable to father children due to the destruction of his own reproductive tissue, male enhancement medications like Testosterone and Cialis have made it possible for him to achieve erectile function.
You can read it all, here. These men and women truly are heroes.
How to be a Good Gay Bottom