What do you think when you hear 'developing' and 'developed' countries?
What pictures come to your head? And how do those pictures compare to reality?
For most people, when they say developing vs developed, what they mostly mean is one of the following -
Poor vs rich
West vs rest
Low-income vs high-income
These definitions were true when they were adopted a few decades ago. But the world has changed a lot in the meantime. There is no such clear distinction left between the developing and the developed nations.
For instance, below picture perfectly captures the difference between the developing and the developed countries.
In the above, x-axis represents the number of babies born per woman  and the y-axis represents percentage of children surviving up to age 5, derived from Mortality rate for babies under 5 years of age . Each country is represented by a bubble and the size of the bubble corresponds to the population of the country .
As you can see the distinction between developing and developed is clearly visible in the above image. This is because back in 1970s the developing countries did not have the infrastructure and funds to support good medical facilities, medicines and vaccinations, proper nourishment of children, etc. And since a lot of babies used to die before reaching to age 5, people were forced to conceive more babies to keep their family running. While in developed nations, because of good facilities mortality rate of babies was too low, so average number of babies required to keep the family tree running were low too.
Fast forward to recent times, as you can see below that a lot of countries have caught up with the 'developed nations' and have significantly improved 5 year mortality rate.
Checkout my Tableau dashboard below to see the improvements happening every year. You can also view a better version here.
The lines between the developing and the developed countries have blurred to an extent that the very definition has ceased to exist.
If your definition is based on income, then every country now has people earning at the same level adjusted to PPP and inflation. If your definition is based on some other measure, it is highly likely that your measure of progress has some causal affect of income.
Its more accurate to compare the people living within similar income levels across the countries instead of labeling entire countries as developing or developed. Check out Dollar Street project from Gapminder.
 - Max Roser (2014) - "Fertility Rate". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/fertility-rate' [Online Resource]
Estimates, 1950 - 2020: Annually interpolated demographic indicators - Total fertility (live births per woman)
 - Estimates developed by the UN Inter-agency Group for Child Mortality Estimation ( UNICEF, WHO, World Bank, UN DESA Population Division ) at childmortality.org.
 - Total population is based on the de facto definition of population, which counts all residents regardless of legal status or citizenship. The values shown are midyear estimates.
( 1 ) United Nations Population Division. World Population Prospects: 2019 Revision. ( 2 ) Census reports and other statistical publications from national statistical offices, ( 3 ) Eurostat: Demographic Statistics, ( 4 ) United Nations Statistical Division. Population and Vital Statistics Reprot ( various years ), ( 5 ) U.S. Census Bureau: International Database, and ( 6 ) Secretariat of the Pacific Community: Statistics and Demography Programme.