Introduction
The phenomenon of population aging was first linked to Europe and quickly became a global issue. Currently, the number of older adults worldwide exceeds that of children under 5, and this trend is expected to intensify in the future [1]. In Iran, population aging is also likely to continue with an increase in the number of older people in the upcoming decades. In 2006, Iran’s fertility rate fell below the replacement level and, after a slight increase in the early 2010s, it continued to decline thereafter. Replacement-level fertility requires that each woman have an average of two children. Positive population growth at the replacement level of fertility is called “population momentum.” Since population momentum, which results from past changes in fertility and mortality rates, particularly affects old age groups, it plays a significant role in accelerating population aging. This study aims to examine the impact of population momentum on the aging of Iran’s population and its implications for social policy. 

Methods & Materials
This study applies secondary data analysis and uses data related to fertility and mortality rates in Iran from 1996 to 2016. The study calculates population momentum and analyzes population aging trends through age distribution. To examine the population momentum, the concept of a stationary population was used. A stationary population is a state of population growth where the growth rate is zero and the birth rate equals the death rate. The population momentum was determined by calculating the ratio of the stationary population size to the initial population size. To calculate the stationary population in this study, each of the studied years was considered as the base year, and the age/sex distribution of the population for the next 100 years was predicted using the cohort-component method of population projection in R software. Assumptions included: a) fertility suddenly reaches the replacement level, b) fertility at the replacement level and mortality at the study year level remain constant during the forecast period. This method allowed for a detailed analysis of the effects of population momentum on age groups and the trend of population aging changes over a 100-year period. This time frame was selected to allow sufficient time to reach a stationary state and fully examine the effects of momentum. The ageing index, which refers to the number of persons aged 65 and over per 100 persons aged 0-14, was also calculated for each year.

Results
The results of this study showed that the momentum of the Iranian population in 1996, 2006, 2011, and 2016 was positive, but its magnitude gradually decreased. In 1996, with a fertility rate higher than the replacement level, the total momentum for women and men was about 1.7; this means that the country’s population, due to the momentum effect, would increase by about 70% until reaching stability. With a decrease in fertility rate in the following decades, the total momentum decreased and reached about 1.3 in 2016.
The age-specific momentum analysis showed that the highest values of momentum were related to the age group of ≥65 years. This value was about 4 in 2016, indicating the high potential for an increase in the elderly population. In contrast, the momentum of the age group <15 was almost constant and negligible, and the momentum of the age group 15-64 had a declining trend and reached about 1 in 2016.
Predictions based on the momentum of 2016 indicate that the population aged 65 and above will reach its peak (approximately 21 million people) by 2060 and then stabilize (with a slight decrease) at around 19 million people by 2116. During this period, the rate of population aged <15 will decrease from 24% in 2016 to 19% in 2116, while the rate of the elderly population will increase from 6% to 21%. Therefore, the aging index is expected to increase from 25 in 2016 to 110 in 2060, and then remain constant at around 100, with a slight decrease. Additionally, the results showed that in the study period, the decrease in the rate of the pediatric population by 2016 played a major role in the increased aging index. However, after this year, the increase in the rate of the elderly population, influenced by momentum and the arrival of large cohorts of baby boomers to elderly ages, will be the main factor in changes in the aging index.

Conclusion
The positive values of population momentum are gradually declining. Age distribution of momentum revealed that the highest values of momentum are allocated to the elderly population. According to the population projection, the elderly population of Iran, particularly influenced by population momentum, is expected to increase fourfold, and the aging index is expected to rise from approximately 25 in 2016 to 110 in 2061. However, this increasing trend of the elderly population will not continue after 2061, as momentum growth becomes zero and then negative due to low fertility rates. Therefore, population momentum plays a key role in accelerating Iran’s population aging. Despite the high rate of population aging in Iran, we face significant challenges in developing social policies for older adults. This situation requires Iranian policymakers to adopt necessary reforms in retirement, health, elderly care, and family-friendly policies.

Ethical Considerations
Compliance with ethical guidelines
This article is based on secondary data analysis. Since no experiments were conducted on animal or human samples, the need for ethical approval was waived.

Funding
This study was extracted from the dissertation of Nazanin Aghaei at the Department of Demography, University of Tehran. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for profit sectors.

Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflicts of interest
The authors declared no conflict of interest.


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