Because the population of a country has no relationship to the age of the country.

If you look at the top ten countries by population in the world, the US is the third oldest after China and Russia, and I'm being generous with China and Russia because those states haven't continuously existed in the same form for as long as the US has.

I'm trying to understand the premise of the question, it almost sounds like you are thinking of it from the perspective of a computer game where every country starts with a set number of people and goes from there or something.

Obviously in the real world when a country comes into existence there is already a population there, which could be large or small depending on what has happened in the past, and a country can increase its population by immigration, or having kids, or in the case of the US kidnapping and enslaving people from elsewhere.