The shape of the age-sex pyramid of the foreign-born population is very different from that of either the total population or native population. There are several reasons for this. First, many migrants leave their home countries to find work abroad, so a high number of foreign born in the economically active ages of 20 to 54 is not uncommon. As can be seen in the foreign-born age-sex pyramid, the majority of immigrants in 2000 were adults between the ages of 20 and 54. Second, in general, children rarely migrate by themselves and adult immigrants tend to migrate with few or no children. This helps explain the relatively small amount of people age 20 and younger. There is another reason, however: the children born in the United States to adult immigrants are considered native and are not included in this age-sex pyramid. Third, people are less likely to migrate at older ages. In the foreign-born age-sex pyramid, there are clearly fewer people in the retirement-age groups (age 55 and over). This low number of older immigrants is also caused by immigrants returning home for retirement and the death of older settled immigrants. All of these factors give the foreign-born age-sex pyramid its “diamond shape,” making it significantly different from the total and native population pyramids.
Migration Policy Institute tabulations of the U.S. Census Bureau's 2000 Census data.