Introduction
After passing the first and second stages of the demographic transition and entering the third stage, the population of Iran is faced with an increasing proportion of elderly people [12, 15, 16]. With relative stability in fertility and mortality, migration is vital in changing population patterns and shaping the regional aging process. Migration by reducing the risk of old age in receiver-migrant areas and increasing its speed in immigrant-first areas has opposite effects on aging [21, 22]. Due to the lack of balanced development in the country and as a result of increasing displacement from underdeveloped areas to more developed areas, migration will have significant effects on the aging ratio of the areas [24]. Therefore, knowing the dimensions of the problem of aging and its spatial distribution can help planners to know and manage it properly. In this regard, this study seeks to gain an overview of the spatial distribution of aging and the effect of immigration on it from 2006-2016. 
Methods
The present study is of the secondary analysis type and was conducted based on the analysis of the data of the general census of population and housing in three periods of 2006, 2011, and 2016 at the level of the country's cities. The relevant indicators used to measure aging are the aging ratio (the ratio of the population over 65 years old to the total population), the aging dependency burden ratio (the ratio of the population aged 65 years and over to every hundred people aged 15-64 years) and for migration, the net migration rate (the difference of immigrants entered to exit to the total population). In this study, the results are presented in the form of Figures and maps. Pearson's correlation and analysis of variance were used to analyze the relationship between the ratio of aging and immigration and to show the effect of immigration on this ratio. Excell and SPSS software, version 22 were used for data analysis and to present the results more comprehensively and dynamically, the spatial distribution of the aging ratio as well as the net migration rate was depicted in the form of a map by Arc Gis software.
Results 
The findings show that the growth of the country’s elderly population has reached more than 5% per year, according to the United Nations criteria, Iran’s population is on the threshold of entering the aging period; however, the share of the elderly population in different regions of the country is not the same, and the cities of the country have different proportions of elderly people. But what is happening at the same level in the country is the increase in the number of elderly cities every year. According to the results, while in 2016, only 19% of the country’s cities had a ratio of more than 7% of the elderly population, this index reached 26.5% and 32.9% in 2011 and 2016, respectively.
The trend of internal migration in the country and the age pattern of immigrants is toward the migration of the young and educated class. As a result of this trend, the immigrant-first cities by losing a high proportion of their young population witness an increase in the proportion of the elderly population, and in contrast to the receiver-migrant areas, by attracting people aged 15 to 35 years, this trend moves toward the youth of the population. To show the effect of migration and aging on each other and its spatial distribution, two indices of net migration rate and aging ratio in different cities were adapted to each other and four categories of cities were created, including young immigrant-first, young receiver-migrant, elderly immigrant-first, and elderly receiver migrant. The relative distribution of these cities in the country showed that during the studied period, young receiver migrant cities always had the highest frequency, but every year, the number of these cities decreased and the number of elderly immigrant-first cities increased so that it has reached from 4.4% in 2006 to 23.3% in 2016. The survey of the spatial distribution of aging also showed that the cities of the northern and northwestern regions of the country have the highest proportion of aging. The maps of migration in the country also showed that the cities of the border regions of the northwest, west, southeast, and to some extent the south of the country lose their population every year in favor of the central regions and the cities of Tehran, Alborz, Isfahan, Yazd Provinces, Iran.
Discussion
 The results show the regional imbalance of aging. The spatial pattern of aging in Iran is clustered. In addition, Iran’s population is transitioning from middle age to old age, which requires investment in these sectors due to the special needs of the elderly in various economic-social, health, etc. dimensions. The border areas of the country, especially the deprived areas, need development programs because the increase in immigration from these areas causes excessive migration to the centers and inner circles of the country. Increasing financial investments, creating economic enterprises, and concentrating educational and recreational facilities are among the plans that should be put on the agenda to reduce regional inequalities to prevent migration. To solve the issues and problems of the elderly, first, it is necessary to have information about the spatial distribution of the elderly population so that appropriate policy and planning can be possible using accurate information. 

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.

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