Explanation:
Puberty normally starts in boys between the ages of 9 and 14. The first sign of puberty in boys is testicular enlargement.

The lack of early pubertal changes (i.e. testicular enlargement) by 14 years of age is considered delayed puberty in boys. Testicular enlargement is defined as a volume 4 ml or greater or the longest diameter being greater than 2.5cm.

Puberty is considered delayed in girls if breast budding is not apparent by 13 years of age.

The Tanner stages (also known as sexual maturity rating, SMR) categorize pubertal development. Tanner stage 1 is prepubertal whereas Tanner stage 5 is adult maturity. In boys, penis and testis development, as well as pubic hair growth are assessed. In girls, breast development and pubic hair growth are assessed. A diagram of the Tanner stages for male genitalia is shown below.

Photos or drawings of the various stages can be found in any pediatrics textbook. The review article listed below provides a good overview of puberty. This article also contains figures on female breast development and pubic hair development in both sexes.

Rosen DS (2004) Physiologic growth and development during adolescence. Pediatr Rev 25: 194-200
http://pedsinreview.aappublications.org/cgi/content/full/25/6/194

Figure 2. Sexual Maturity Rating for male genitalia. Stage 1 describes prepubertal genitalia. In stage 2, there is enlargement of the testes and scrotum, with reddening and thinning of the scrotum, but no enlargement of the penis. In stage 3, the penis begins to enlarge, first in length and later in diameter. The testes and scrotum continue to enlarge. In stage 4, the testes and scrotum continue to enlarge, with continued lengthening of the penis and enlargement of the glans. Stage 5 represents genitalia of adult size and proportion.
Reproduced with permission from Pediatrics in Review, Vol. 25, Pages 194-200, Copyright © 2010 by the AAP.