Marriage age plays an important and influential role in family formation and childbearing. Investigating changes in attitudes toward the appropriate age for marriage among young people can be effective in predicting future changes in the country's population and developing and implementing accurate and intelligent programs. This study, using secondary analysis of three waves of data from the National Survey of Iranian Values and Attitudes, seeks to monitor changes in attitudes toward the appropriate age for marriage among young people during three periods: 2004, 2015, and 2017, and to explain the contribution of effective factors and their differences over the past three decades. The findings show that during each period of one year and over the past three decades, the appropriate age for marriage for girls and boys has increased by approximately two years. By controlling for the research variables of employment status, socio-economic status, and educational status, there was a significant effect and difference on the age for marriage among young people. Over the past 30 years, education has been a key variable with an increasing contribution to the appropriate age for marriage for girls and boys. Single people, aged 30 to 49, employed, and with a diploma or higher education have contributed significantly to the increase in attitudes towards the marriage age of young people in recent decades. Therefore, effective policies and planning should seek to change family-oriented values and attitudes through cultural transformation programs and simultaneously improve economic conditions to encourage young people to start a family.