That would be a "young artists, yeah, younger fanbase, no".
Fans of the books as canon and films are probably younger. The HP fandom, that is, the slash and fiction writing part of it, is primarily adult and has been for years.
For some reason, HP and anime fandoms tend to draw more artists to them. And since there are no gay characters in HP (cept for Dumbledore and Grindelwald), as you say, they had to create them. When you have canonical gay characters, you have all the images you could ever want. Yaoi, of course, is gay characters. But there is a lot of anime where the characters are slashed, so again you have artists scrambling to fill the void.