The theory of Demographic Transition stipulates that the number of countries with the replacement and sub-replacement level of the total fertility rate is growing. In these conditions, population dynamics and the sex and age composition are increasingly affected by migration. The above holds true for Eastern European countries. Population decline has haunted Latvia for two and a half decades. Since 1990, the net migration rate has been negative, which contributes to depopulation. This study aims to reveal the effect of migration on the sex and age composition across Latvia and its largest cities. The authors consider hypothetical transformations in the country's age structure in 2000-2015 in the case of zero net migration. The study uses the cohort component methods and considers the actual age-specific mortality and birth rates. The analysis of the results obtained for the population of Latvia and its individual cities makes it possible to identify temporal and age/space features of migration. A comparison of the official data with net migration rates calculated for different age groups ensures a more accurate estimate of the actual volume and direction of migration flows for certain Latvian cities. The method for calculating net migration for the selected age groups, described in the article, may narrow the gaps in the current migration statistics and reveal the territorial inhomogeneity of demographic processes.