Age, any particular period in the existence of organic beings, of collective humanity, of nations, or of the globe. The age of the world has been variously computed by geologists, but nothing positive is known of the real length of time allotted to each period, so strongly marked by changes in the structure of its crust, and in the forms of animals and plants which have left fossil traces of their existence. Many periods of inorganic change, and numerous mutations of animal and vegetable forms of life, are known to have occurred upon our globe before the slightest trace of man appears upon its crust; and hence it is inferred that human life, compared with the inferior forms of animal and vegetable life, is of comparatively recent date. The age of the world, then, has two distinct bearings, one referring to the origin and growth of the earth in its cosmological and geological existence, the other to the origin and history of man and of society upon its surface. Certain periods remarkable for some particular development in the life and progress of the race, or of a nation, are distinguished by particular names, such as the golden age, the silver age, the copper or the brazen age, and the iron age of heathen mythology, the Augustan age of the Roman empire, the Elizabethan age of English history, and the age of steam and iron in the progress of our own time. (See Ages.) - The life of man has been divided into seven ages by Shake speare, and into four, five, six, seven, or eight, by men of science.

Some make four distinct periods only, such as infancy, youth, maturity decline; others follow more closely each physiological transition, and divide existence into infancy, childhood, boyhood or girlhood, adolescence, virility, maturity, decline, and old age or second childhood. The most natural divisions are those which distinguish the ascending the culminating, and the declining periods of life. Each of these may be further subdivided according to physiological changes which mark the transitions from one period to another. Infancy, childhood, and boyhood or girlhood mark the first stage of ascending progress from birth to puberty; youth, adolescence, and manhood or womanhood mark the second stage of ascent in the growth and evolution of the powers of life; virility may be applied to the culminating period; and the descending stages of maturity and decline might well be subdivided into lesser and marked periods of transition, as are the two ascending stages. Infancy applies to the first two years of life, during which the first complete set of teeth is developed; childhood to the age, between 2 and 7 or 8, when the first teeth are shed, and a more complete set replaces them; boyhood and girlhood from 7 to 14 or 15, the average time of puberty, which forms a marked transition, closing the first general phase of ascending progress.

Here youth, properly defined, begins, and lasts until the age of 20 or 21, when the physical development becomes complete; the bones are firmly set in all their parts; the mind is also more or less developed; and the sexes have attained "majority " in social life. Adolescence is applied to the first period of adult life, from 21 to 28, and manhood to the riper period, from 28 to 35 or 36. The culminating period of physical and mental force combined is termed virility, and this may vary in different individuals, some waxing feebler soon, while others retain all their vigor from 86 to 48. The body then begins to lose its energy, and gradually declines through the descending periods of maturity and old age. The mind may still retain its power, and even acquire more knowledge and experience, but the body will not maintain so vigorous an exercise of thought and nervous action as in former years. The subdivisions of descending life are not so strongly marked, apparently, as those of the ascending phases; but in woman's life there is a critical period called "the change of life," which corresponds inversely to that of puberty. The capability of child-bearing begins with one and ends with the other. The " critical period," however, is not so fixed as that of puberty.

With some it occurs at 40 or 42, while with others it extends exceptionally to 50, 55, or 60, and in some rare instances still later, the average being 45. This period of sterility is less marked and regular in man than in woman. Whatever be the length of the descending phase of life in different individuals, the ascending periods are nearly uniform in their average duration. The female sex is usually more precocious than the male, and women average longer lives than men; but that is probably because they are less exposed to accident and danger in the common course of things, for the extreme cases of old age recorded are more numerous in males than females. - Individuals become legally qualified for certain acts at given ages, and these vary in different countries according to the laws and institutions. A child under 10 1/2 years of age is not amenable to the laws of England for serious offences; the parents are responsible for its actions in minor cases. Above that age the offender is responsible, when deemed competent to distinguish between right and wrong. The age of 14 is fixed by the civil law as the age of criminal responsibility; capital punishment, however, was inflicted for murder in 1629 on a boy of 8 years of age, who had most artfully concealed the body of his victim.

The oath of allegiance may be taken after 12, and youths of either sex may choose a guardian at the age of 14; but no person under 21 can execute a valid will. The nubile age was fixed by the Roman law at 14 for males and 12 for females, and at these respective ages either sex may in England consent to marriage, with the approval of guardians. By the Code Napoleon, the nubile age in France is 18 for males and 15 for females, with the approval of guardians; at 17 a person of either sex may be an executor or an executrix; and at 16 a minor may devise one half of his property. In the United States either sex may choose a guardian at 14; the nubile age is 14 for males and 12 for females. The age of majority, which gives both sexes the free disposal of themselves and their property, and the enjoyment of all civil rights belonging to their sex and condition, is 21 in the United States, Great Britain, and modern European countries generally; but in ancient Rome minority continued till the age of 25. - Some philosophers believe that, under well devised rules of conduct and favorable external conditions, the natural period of human life might be extended to 100 years, in lieu of the " threescore years and ten " of the Bible; and M. Flourens, a French physiologist of high standing, has published a work in which he treats of " human longevity" as dependent upon human prudence mainly, and easily prolonged by care to the limit of 100 years.