I'm well aware that such a date is arbitrary, but although I'm familiar with some other cut-off dates such as 1453, 1492 or even 1481 (which is perhaps convenient for studying the history of France but not for anything else), I don't remember having see the 1520s mentioned until earlier today when I read the author's note of How to slay a dragon.
How To Slay A Dragon was written by this subreddit's own /u/sunagainstgold so she would be able to explain her point about why "the Middle Ages ended in the 1520s" (note that she states "I am unassailably correct about this Unassailably" - which implies there some archness in her statement). My assumption would be that it has to do with the rise of Protestantism and Martin Luther - but she would be the best person to respond.