British physician JM Tanner described five stages of the development of puberty in boys and girls.
The stages in boys are described and illustrated here. The range of variation in age for each stage is wide, and the development of penis and pubic hair may not take place in step - a boy can often be at stage 3 or 4 in penis development and still at stage 2 for pubic hair.
Stage 1. Juvenile. Age range 0 - 14, but typically up to age 9. Testicle volume less than 1. 5 ml, penis 1 - 3 cm in length, often pointing forward rather that hanging down. No pubic hair. Spontaneous erections rare or absent.
Stage 2. Age range 9 - 15, but typically 9 - 11. Testicle volume up to 6 ml, scrotum enlarges and reddens, penis length little changed, minimal straight pubic hair. Spontaneous erections become frequent.
Stage 3. Age range 10 - 16, but typically 11 - 13. Testicle volume 6 - 12 ml, scrotum enlarges further, penis length up to 6 cm but girth only slightly enlarged, pubic hair begins to become coarse and curly. Ejaculate, if any, small drop of clear fluid.
Stage 4. Age range 12 - 16 but typically 13 - 14. Testicle volume 12 - 20 ml, scrotum enlarges and darkens, penis length up to 10 cm erect, girth increased, pubic hair adult in form but not in extent. Full ejaculation.
Stage 5. Adult. Age range 12 - 17 but typically 15 - 16. Testicle volume 25 ml, adult scrotum, penis length around 15 cm erect, pubic hair adult in form and distribution.
Marshall, W.A. and Tanner, J.M. (1970) Variations in the Pattern of Pubertal Changes in Boys. Archive of Diseases in Childhood, 45, 13-23. Full text