I think that if I had never discovered metal through discovering such amazingly unique metal bands like Arcturus, Opeth, Maudlin of the Well and In the Woods there would be a high chance I would eventually grow out of it. Say I'd only found bands like Slayer, Death and Emperor (which are all actually bands I really like
) I'd probably one day grow out of the sheer-aggression of such acts. But now that I've fully explored the genre of metal, from all angles, and discovered incredible bands of all shapes, sizes, colours and tones...some intense, some softer and more artful, I really can't ever see myself fully moving away from it. Metal has about 10X the amount of quality to offer the average joe who simply thinks it is 'just noise'.
IMO, most people that 'grow out' of metal do so because they simply have NOT explored the genre sufficiently. They get into nothing but aggressive metal (and not artsy or more progressive metal that twists the metal 'norm'), and upon finding their tastes softening, move onto something else. People who do this are probably just trying to be cool or 'alternative' for a bit, or are in a process of mental maturation, and put too little effort into actually discovering that metal is incredibly diverse, and more than just 'aggressive noise'. They want a quick fix of anger and that's all they let themselves have, before moving on.
Myself, I've simply found out how much it can offer. I've got my extremely aggressive metal (Slayer, Kreator, Nile), which I can temper and mix with something more progressive (Opeth, Maudlin of the Well, Atrox), to get a full listening experience out of metal. Knowing all that I know now about metal, growing out of it would be a waste of a few years of my life and something that just isn't going to happen...possibly ever.