Dear Addis Girma:

I'm sorry to say that there are no standard population conversion factors. Populations have different demographic characteristics. As a result, the proportions of the population made up of the groups you mention in your question differ between different populations. This is especially true of displaced and emergency-affected populations In which different demographic groups may flee and different groups may stay. Displaced populations are often quite dissimilar from the settled populations from which they come.

In the past, certain rules of thumb were commonly used, such as the assumption that 20% of a population consists of children under 5 years of age. Such rules of thumb have become increasingly invalid over time. For example, there is to my knowledge no population on earth with the fertility rate high enough to result in 20% of the population being preschool age children. 

Nonetheless, there are multiple sources of demographic data on national and subnational populations. Chapter 2 of the final reports of Demographic and Health Surveys almost always contains a table, often Table 2.1, which shows the age and sex breakdown of the members of urban, rural, and total households contained in the survey sample. Unfortunately, the reports of the Multiple Indicator Cluster Surveys from UNICEF do not contain comparable Information. 

In addition, the government bodies responsible for census and/or statistics in many countries have websites containing demographic reports. Such reports often will give detailed demographic breakdowns for national and subnational populations. For many countries without such websites, the US Census Bureau has a database containing census data from many countries (see: https://www.census.gov/programs-surveys/international-programs/about/idb.html). 

I would encourage you and others to avoid use of 'standard population conversion factors' and to search for population-specific demographic data to ensure that you use the most accurate estimates possible

The only popluation I'm aware of that meets this rule of thumb is Niger who has almost exactly 20% < 5s. 

https://www.census.gov/data-tools/demo/idb/#/country?YR_ANIM=2021&FIPS_SINGLE=NG