The average penis size is 5.6 inches and will generally stop growing at about age 21. You will normally have a ''growth spurt'' at around age 13-15 and it will continue to grow slowly. This is AVERAGE, people will vary. A guy on the tv show 'Extreme Male Beauty' (Channel 4 UK) found that using a technique called Jelqing (nothing but your own hands required: Google it)he enlarged his penis by an inch.
My boyfriend is 7.5 inches and I wouldn't want him to be any bigger than this. If you're worried about your size, perfect your cunnilingus and no girl will care!!
P.S. I think people who are saying 12 and 13 inches are getting confused with cm and/or trying to impress people on an annonymous forum!