For my setting, I really want to push the non-human aspect of the other typical fantasy races. For Elves, I've been throwing the idea around that they go through a metamorphosis of sorts. I was thinking that they would look passable as a human to about age 15-20, then go through their metamorphosis. They would gain the typical appearance of an elf in my setting, ie tall, lanky body, sharper features, larger and pointed ears, etc.
This would obviously be very painful, so I was thinking during this metamorphosis, they could gain a leather-like "covering" that grows over certain areas of the body, such as neck, belly, top of arms, etc as protection, which would dry out afterwards into a birchbark-like material that can be torn off. The covering could grow to roughly appear like surfaces near to them, such as bark, dirt, etc.
Why would Elves go through this at all? I played with the idea of them being a race that replaces human children with their children (or that sends Elven children to live with humans), but I don't think that would work for my setting because I want Elves to have a central society that does raise their own children.
I should note that in my setting, Elves are magical creatures, so it doesn't necessarily require all answers to be 1000% grounded in realism.