Actually, from what I've read in three different studies, most (about 85%) males attain their full adult penis length by the time they reach sixteen years of age. There are exceptions, and the number of members here who have experienced penis growth beyond that age attest to that. Perhaps this extended period of growth has something to do with the larger than average endowments of many of them. I fit into the more usual scenario: my cock was full-sized by the time I reached my sixteenth birthday. Keep in mind that at puberty primary sex characteristics usually begin developing earlier and reach their full potential before the secondary sex characteristics, such as height. In general, a teenager's penis completes its growth before he reaches his full body height.