What is a curmudgeon?

One definition is "a crusty irascible cantankerous old person full of stubborn ideas." I like the stubborn ideas part, and can live with being crusty, irascible, and cantankerous, but take issue with the word "old." Curmudgeons do not have to be old, as I can attest. One can be a young curmudgeon, although some might prefer to call such a person a contrarian or an iconoclast or just a pain in the ass. The word "coot," on the other hand, almost always is accompanied by the word "old," so I cannot claim to nor aspire to be an old coot. Just a curmudgeon.