I got measured myself for the first time when I was only 11 years old when my horny step sister wanted to do it and I was a little over 9 inches then,
She had overheard someone talking about how long an average penis is... (her mom, my step mom I think my step sis overheard) she knew already that I was larger then that even back then, as we often swam and played nude at our pool together and that was when her mom also seen me and my brother nude.
We watched each others bodies as we went thru puberty.
and again we measured it together (my horny step sis and I ) when I was almost 16 yrs old.... and I was now the same then as I am still at now... at 10 1/2 inches,
But I do know that I got alot thicker as I aged.
So I believe that in the most cases that you have it all as a young adult and thats all you get!
I have been measured by my wife also, (now ex wife after 10 years of it) and she used to tell everyone that asked that I was 11" measured along the top of my cock shaft from my hairline to where it attatches to me, but she was pulling a bit on it too!
Grant