ANDY'S ANSWER: Because boys are going through the different stages of puberty at different times, it's really difficult to say what an average penis size should be for a 16 year old. It really depends on how developed you are. Some boys may be close to being fully developed and some may still have some growin' to do.
Boys can be still growing up to age 20 or more. The best I can do tell you that the average size for an adult penis is usually somewhere between 5 and 6 inches. Regardless of the size, as long as it functions as it is meant to function, it is normal.