Pubertal development. Penile length measurement is rarely used to monitor pubertal progress. The timing of puberty differs in each individual. Tanner stage 5 is characterized by adult genitalia. To answer your question, though, mean penile stretch length (non-erect) by late puberty is around 3.75 in, for white males. Mean erect adult human penis is between 5 and 6 inches in length.
6cm-14cm avg range. These lengths are taken from a table in the textbook adolescent health care: a practical guide. These are approximate ranges of nonerect (soft) penises. Age 14: 2.4 inches (6 cm) to 5.5 inches (14 cm) it is important to note the genetics plays a major role in all physical expressions during development such that variability is high from one teen to the next.