There are books you can look in if you really want to know.
When I was in sixth grade I found a library book, The Biology of Adolescence, that graphed median penis measurements for ages 2 through 25. I don't remember all the figures, but it showed adult size to be 14-16cm and since I measured 15 cm I thought my penis was done growing.
Gary Griffin published a book (I don't remember the title) that included cumulative frequency charts in 10% increments over the same age range, probably graphed from the same data. Sorry I don't have the title or the data, but that's the book to look for if you really want details.
Donald J. Templer's book Is Size Important lists results from a 1942 American study and a 1964 Japanese study. In the American study, average length increased from 2 1/2 inches at age 10 to 5 1/8 inches at age 20. The Japanese study reported an increase from 1 3/4 inches to 3 3/4 inches over the same age range. (The studies aren't necessarily comparable.)