[Andrés]

Where do the holy scriptures say that this is the only inhabited world anyway?
God could have created billions of alien races in His likeness.
Why would have God created such a vast universe to place life only in this tiny little insignificant rock.

About the age of an alien civilization, remember that human civilization has existed for a tiny fragment of Earth life. For example what would have happened if dinosaurs would have survived and had the chance to evolve into a sentient race. The point is that an alien civilization could have evolved long before humankind or long after. Also consider that earth like worlds were not created all at the same time, some may be much older and others younger.
It is more plausible that an alien culture is either many millions of years ahead or behind us that in a similar cultural age.

IMHO it is very plausible that many alien races are more advanced than us.
It is also plausible that some of them could have already created ships like those I had described a few post ago.
It is very unlikely that any of those ships will reach Earth anytime soon.

Right why is it that according to most sci-fi a race = a nation?
Most likely an alien world would have many countries with a complex history, and many wars, just like us.
Is evolving into a single culture a requisite for reaching space-age?