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The sex structure^{1} or sex distribution^{1} of the population is generally measured by the ratio of the total number of one sex^{2} to the total number of the population, or to the total number of the other sex. When the male sex is considered, we often speak of the masculinity^{3} of the population. The masculinity proportion^{4} is the proportion of males in the total population. The sex ratio^{5} is the ratio of the number of one sex to that of the other; the ratio of the number of men to the number of women is called the masculinity ratio^{5}.
 2. sex n. — sexual adj.
 3. masculinity n. — masculine adj.
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The terms males^{1} and females^{2} are commonly used in demography in place of men^{1} and women^{2} to refer to persons of each sex at all ages including boys^{3} and girls^{4}, The word man^{5} is, however, also used in the sense of human being^{5}.
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Age^{1} is another fundamental characteristic of population structure (1444). Generally it is expressed in years, or years and months, save in the case of very small children, where it may be given in months and days only, or in years and decimal fractions of years. Statisticians often round off the age to the number of complete years^{2} lived, and this is called age at last birthday^{3}. Where the fraction of the last complete year lived is counted as a whole year, as in some actuarial applications, we speak of age next birthday^{4}. Occasionally age at nearest birthday^{5} is given, where the age is rounded to the nearest integer. The term exact age^{6} is used, particularly in life table calculations, to denote the date when an individual reaches his birthday. Thus, a man will reach exact age 25 on his 25th birthday, and a man aged 25 will be between exact age 25 and exact age 26.
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In demography, certain terms which have been taken from everyday language are used to denote different age periods^{1}, though without precision. At the beginning of life comes childhood^{2}. In general a child^{3} is a person who has not yet attained puberty (6202). In the very early days of life, the child is called newborn^{4}. A child at the breast^{5} is a child who has not yet been weaned from its mother. The term infant^{6} or baby^{6} may be used to denote a child who has not reached his first birthday, though in colloquial language it may be applied to slightly older children. Children who have not yet reached the compulsory school age are called preschool children^{7}, a school child^{8} is a child who habitually attends school.
 6. infant n. — infancy n., the period of being an infant —. infantile adj. In legal terminology, an infant is a person who has not yet attained his majority, generally a person under 21 years of age, who is often called a minor.
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Childhood is followed by adolescence^{1} which is marked by puberty (6202). An adolescent^{2} is a person, who is entering adolescence. The terms young persons^{3} or juveniles^{3} are employed for men and women approaching adult age^{4}. Those who have reached maturity^{4} are called adults^{5}. Old age^{6} is frequently taken as beginning at the age at retirement^{7} (3613) i.e., about 60 or 65 years. Persons above that age are called old people^{8}.
 3. The term youth is employed for a male young person, and also for the age period referred to.
 4. maturity n.  mature adj.  maturation n.,the process of growing to maturity.
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The age distribution of a population is either given by individual years of age^{1} or by age groups^{2}, which may be quinquennial age groups^{3} or quinary age groups^{3}, or broad age groups^{4}, such as 019 years, 2059 years, 60 years and over. Occasionally a population’s age distribution^{6} or age structure^{6} is given by classifying the population by year of birth^{5}. Graphically an age distribution may be represented by a population pyramid^{7} which is a histogram (1558) showing the population by age and sex and so named because of its pyramidal shape.
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The mean age^{1} of a population is the average age of all its members, the median age^{2} is the age which divides it into two numerically equal groups. When the proportion of old people in a population increases, we speak of the ageing^{3} of the population. An increase in the proportion of young people may be called rejuvenation^{4} of the population. The term ageing used above should not be confused with the technique of ageing^{5} a population. This technique consists of applying certain ageing factors^{6} to particular age groups of the population, in order to compute the total number of survivors of that age group at a later date.
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Ageing (3265) of a population must also be distinguished from individual ageing^{1} or senescence^{1}, and from an increase in the duration of human life or increased longevity^{2} which is the result of improved standards oi living and of medical discoveries. An individual’s physiological age^{3} will depend on the state of his tissues and organs. In the case of children we speak of mental age^{4}, which is defined as the age at which the attainments of the individual child as measured by certain tests can be performed by the average child. In studies of mental and physiological age, a distinction is made between these ages and chronological age measured by the time elapsed since the individual’s date of birth^{5}. The ratio of mental to chronological age is called the intelligence quotient^{6} (often abbreviated to I.Q.).
 1. senescence n. — senescent adj.
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