When the demographic transition started in the 1970s in Thailand, internal migration of this country also manifested a turning point. Newly reclaimable vacant land almost vanished in the 1980s, although surplus populations in rural areas were being absorbed by other rural areas with lower population densities until the 1970s. This study analyzed population and internal migration changes in Thailand from 1980 to 2030. Analysis of populations by age group in urban areas (central region and Bangkok metropolis) and rural areas (northeast region and Surin province) indicated that decreased growth of younger populations in urban areas caused a marked increase in migration of young generations as the labor force moved from rural to urban areas, especially after the 1990s. Statistics of the inter-regional migration census from the northeast to central regions also indicated that this population flow began to increase around the year 1990. The present trends of population aging and various socioeconomic situations in rural and urban areas in Thailand will result in acceleration of population aging in rural areas through deprivation of rural young populations by urban areas.