There's another way to answer the question about the whereabouts of all the aliens, which is to say, they're already here, we just don't realize it. If there really is a civilization, or many civilizations, out there that are a billion or so years older than us, they could very likely move around undetected. Assuming that just because we can't see or hear them with our feeble technology it means that they're not around could be akin to standing on a hillside using semaphore while everybody around us is snapchatting on their smart electronic devices. Just because nobody waves their arms back at us doesn't mean they're not here; it means they're too busy staring at their devices to see us.
Or maybe they do see us, but they're observing us without revealing themselves. We might be, for them, zoological curiosities worthy of scrutiny but not intervention. If they're in "Star Trek" mode, they could be following a Prime Directive protocol, which prohibits them from interfering in the affairs of primitive civilizations such as our own.
Alternatively, they just don't care. We're an inconsequential nothing in a remote corner of an ordinary galaxy. Theoretical physicist Michio Kaku has suggested that we might be akin to an anthill in Peru at the time Pizarro tramped by on his way to subjugate the Incas. In other words? Irrelevant. Along the same lines, they could have already come, seen, gotten a look at our sorry simian state and left. Maybe they showed up a few million years ago, checked out the tortoises and giant ferns and decided to move on?
Or maybe these advanced aliens have transcended mortality and even material existence. They might dwell in some numinous Shangri-La so far removed from the sweaty concerns of our sphere that bothering to communicate with us would be a waste of time so laughably pointless it wouldn't even cross their celestial minds.
Another less benign scenario is that we're nothing more than a holographic simulation, or possibly a game designed by an alien super-intelligence who is currently roaring with laughter at our foibles, or has long since grown bored and walked away, leaving the simulation running. In which case, it's just a matter of time before the off switch cancels us or the batteries run out. Maybe there used to be multiple intelligent life forms in the game, but the other species figured it out and got pulled, which would explain our current solitude.