Update: I just re-watched The Avengers (the 2012 movie). At the very beginning, we see The Other telling Thanos (who we can't see yet - we only see him in the after-credits scene) that the Tesseract is located on "a human world." Not "the human world," "a human world" - implying that there are other human worlds besides Earth.
This means that the accepted answer to this question is no longer valid, although I will keep it checked until a new answer is provided.
In Guardians of the Galaxy and Vol. 2, we run into several humans who seem to have been in space for quite a while, prior to the events of the Infinity Saga (i.e. before 1995).
We know how Peter Quill ended up in space. But there is also Stakar Ogord and Aleta Ogord, who as far as I can tell are humans, and also seem to have been in space for quite a while. Moreover, most aliens seem to be familiar with humans, even though according to the main story-line, very few humans had left the solar system or had contact with extraterrestrials prior to the events of The Avengers (2012).
Have there always been "older" space faring humans in the MCU?