Any good biology book should have a section on human growth and development. This section should give appropriate age ranges and significant developmental characteristics. While age varies, there are some general numbers that can be given as age ranges. Here are the broad descriptions that delineate human growth and development after birth:
0 to 2 years: infancy; baby learns to crawl, stand, walk, master muscular coordination.
2 to 10 years: childhood; child refines muscular coordination, grows taller, may experience "growing pains" as bones in skeleton lengthen.
11 to 13: onset of puberty, where sexual organs mature. Sperm production begins in boys, menstruation and ovulation in girls.
14 to 21: adolescense; sexual characteristics mature. Boys add more lean muscle mass, voices deepen, hair production on body. Girls hips widen, breasts develop.
21 to 40: young adulthood, most men and women are at their physical peak.
41 to 70: middle age years, declining lean muscle mass, acquisition of fat deposits, bones start to lose calcium.
71 and beyond: old age years; continuing lean muscle mass, more susceptible to disease and afflictions healthwise. At some point, death.