What do you mean by fast-aging? Are you talking the population biologically ages to the point of becoming a bunch of geriatrics in just a couple decades?If a nation's power is it's military (though I think it's the economy) and the population in the states is a fast aging one....then what happens if there is a war 30 years from now? A really bad one?
As for a nation's power, it has always been manifested in the military. The economy is fueled by circulation of monetary value. That requires negotiation with outside parties (no matter how self-sufficient a nation is, there are certain raw materials that post-Industrial Revolution societies require that are rare, or not found in the local region) for international commerce. Monetary value basically comes down to territory. Think about it this way: oil is considered of great value, and it is concentrated in certain regions in the Earth. Gold, diamonds, and Silver, have mostly been harvested to rarity or nonexistance in the United States, so we must turn to the biggest stockpile (Africa) to get them.
The economy is fueled by the availability of these resources, compared to how much you can overcharge your customers for the finished product. Control of certain territories is necessary to keep one's economy, and therefore, one's nation, thriving. To achieve that grip on territory, negotiation is required.
Violence is just another tool for negotiation. War is just a society-supported final tool for achieving economic superiority. Competition is natural for living things, and those who have the strength and intellect to gain more territory, which equals more resources, deserve to outbreed the weaker variants.