I believe Valonyx is concerned that younger nations are going to suddenly claim themselves to be more powerful.
while I agree that going by NS's population will be highly unrealistic, I believe Valonyx's concern is that if players get to choose their own populations, they'll claim huge populations and huge tracts of land.
I'd like to point out right now that in real life, the nations with the most political power have next to no land, and miniscule populations. our current world puts the largest countries as the most powerful, and the smallest countries as insignificant, which to me is failing to acknowledge the complex forces at work. i'm babbling. let me stop babbling, and list some things.
firstly, population size and land size have no correlation. there will be large countries with low populations. there will be small countries with large populations.
secondly, neither population size nor land size correlate to political power.
thirdly, hating on younger nations and forcing them to play weaklings just because they're new to gameplay is.... kinda elitist, and a bit rude. people should justify their own strength with their issues, with careful research, and with explicit worldbuilding. not with how old their nation is.