When in the middle ages? See my answers here from the AMA this past weekend about the development of knights. A farmer-knight in 950 was significantly different than the aspiring aristocratic class we see in 1300. From the former, one had a shot at being a knight - but it wasn't a glorified position, certainly not what we come to know centuries later where the average person (an enserfed peasant) had virtually no shot.

Have a read a let me know if there are follow up questions.