Life on Earth is umpteen squillion years old and has been observable via spectrographic evidence for at least a billion years. (Actually, atmospheric composition has changed dramatically over that period, but has probably been significantly-different-from-dead in various ways for all that time.)
Intelligent life on Earth is only about 10000 years old and has only been remotely observable for a couple of decades, depending on where you live. That's assuming that you can detect the radio waves. (And yes, you won't be able to separate individual stations, but you probably can detect the bump in the overall power spectrum, and if you are an alien scientist you can probably recognise that those frequencies match the transmission windows for our atmosphere.)
In another hundred (perhaps) or thousand years it is quite possible that we will be a dead planet in term of radio emissions detectable at long range. Technology changes and broadcasting at high power in all directions is a pretty dumb way to communicate so we are already using alternatives. Then there is the possibility of a completely new mechanism that we cannot guess yet. (I am reminded of the anecdote about the anthropologist asking a neolithic man about his ideal means of communication and being told "a really big drum" but all the while radio waves are passing through his head.)
Life on other planets presumably follows a similar historic trajectory, but plus of minus a few hundred million years!
In a few hundred million years time, we will either be extinct or able to visit planets and reside there without the inhabitants being aware of our presence.
So the aliens are either already here, unnoticed and reading this comment with wry amusement, so they are so far behind that when we visit their planet in the relatively near future we will not be noticed by them. Either way, the technology of the visiting civilisation will be so far advanced of the hosts that there will be no question of "colonisation" or "invasion" because the host planet will have nothing to offer except scientific interest (best served by observing incognito).
The geologists have a term for this; they call is Deep Time (capitalised) and it is the temporal version of the Douglas Adam's paragraph that begins "Space is big. ...".