Hi The average penis size worldwide for men is about ″ when erect. At 15, you probably are still growing, and will continue to do so . At some point, you may have wondered: What is the average penis size? Research says the average length is inches for a flaccid penis and inches for an erect penis. May 10, · Best Answer: depends on the age of the boy The average size at the beginning of puberty is ( in) with adult size( in - 6 in) reached about 5 years laterStatus: Resolved.