The absence of evidence isn't evidence of absence. We've only been "looking" for life for ~50 years!
> It could be that almost all civilizations suffer an extinction event before they get technologically advanced enough to prevent it or survive it.
It seems to me that we've got three possible scenarios here:
Scenario A: There is NOT a Great Filter. In this scenario, we have a large number of civs with a high median age. Scenario B: There IS a Great Filter. It catches between 0-99.99% of civs. In this scenario, we still have a large number of civs with a high median age. Scenario C: There IS a Great Filter. It catches 100% of civs. In this scenario, we have a small number of civs with a lower median age (some ${Civ Age at which Filter Occurs} / 2)
It's hard for me to imagine that we're living in Scenario C. For a Great Filter to catch 100% of civs, it would have to be baked into the laws of physics!
Even then, the Great Filter would have to occur VERY early in technological development for the odds of technological parity to be good. And, of course, an early-acting Great Filter means fewer extant civilizations at any point in time, lowering the odds of an encounter occurring in the first place.
It could be that the only ones we'd actually bump into are those who built and launched generation ships not too long after they became capable of it, which would be when they are not too far ahead of where we are now.
Right now we just don't know enough to say what it is like out there. We do know that we sure haven't been able to detect any signs of alien civilizations. But we don't know if that is because very few civilizations start, or they start but something kills them before they start doing things we could detect, or they are only at a stage where they are doing such things for a relatively short time and then they switch to technology that we can't detect.