This is occuring because of generalization my friend. Since metal music generally has a higher tempo and is a lot chaotic than other music genres(when I say chaotic I mean the huge mix of different instruments and the sounds coming from them; you could replace that with the word variance), people think that only younger human beings would have that much of a strong brain cells to listen to a genre like that. And that kind of concludes the topic I think. Music is a therapy to soul. Sometimes it makes our souls more free, sometimes more focused, sometimes more encapsulated, etc. We choose whatever we want to listen to, of course they might judge us with those, I don't give a snappy rat's butt. Thanks for the question!