Yes, it is scientifically plausible for another civilization to exist in the Milky Way that could be several billion years older than humanity. The Milky Way galaxy is estimated to be around 13.6 billion years old, and our own solar system is relatively young in comparison, at approximately 4.5 billion years old. This means that there has been a substantial amount of time for other civilizations to potentially develop and evolve within our galaxy.
The possibility of advanced civilizations that have had a head start of billions of years raises the concept of the "Fermi Paradox," which questions why we have not yet encountered or detected signs of extraterrestrial intelligence despite the vast number of potentially habitable planets in the galaxy.
There are several hypotheses and possible explanations for the Fermi Paradox, including the possibility that advanced civilizations may exist but are too far away for us to detect, or they may exist but use technologies or communication methods that are currently beyond our understanding.
It's important to note that the existence of extraterrestrial civilizations remains speculative at this point, and we currently have no definitive evidence of their existence. However, given the vastness of the universe and the sheer number of potentially habitable planets, the idea that there could be civilizations that have evolved long before us is considered within the realm of possibility.