In terms of how long the universe has existed, we haven't even been around a second. The first time that we opened our collective mouths and shouted at the universe was a little over one hundred years ago. There are something like 15,000 stars within that volume. First you would have to assume that life existed within one or more of those 15,000 stars. Then you would have to assume that such live was intelligent. Still further to that you would have to assume that such life had not only achieved a technological civilization, but a level of technology that would allow them to travel to this planet via some faster than light method of transport like space folding or wormholes. Still another assumption would be that they would even be able to hear our tiny shout among all of the background radiation, pulsars, and the like given the distance involved. Consider trying to pick out the light of a candle that's held in front of the sun.
That is, of course, also assuming that they didn't randomly just happen across this little planet, on the outside of a galactic spiral arm, by random chance during the relatively short period of time that we've been on it. Estimates place the age of the earth at roughly 4.5 billion years (unless you're a Bible scholar, that is). Life has been around since after Earth's first billion years or so but was essentially single cellular in makeup, until about a billion years ago. The dinosaurs lived from 250 million years ago to about 65 million years ago.
Hominid creatures appeared around 4.5 (some think as long as 6) million years ago. Something recognizable as human has been around for maybe 100,000 years. The first recognizable civilization began in Sumeria, roughly 4,000 years ago.
Do you begin to see what the odds are that some highly advanced civilization has visited Man in the past?
