Sub level 80 characters can do fractals and are upscaled to level 80 when doing them. This can expose new players to fractals, make fractals fun and challenging while running a lower level alt for experienced players, and can be a method to gain exp and rewards while leveling an alt. Scales 1-19 don't have any agony requirements, so it shouldn't be that much of a problem for sub 80 characters who can't get agony resistance gear. Player discretion can be used for level 80s who don't want to party with lower levels, just like dungeons. Alternatively, you could make a separate lfg category.