The skin of new-borns is so delicate its nearly transparent. Fresh into this world, all full of promise and delight, babies are the purest form of being human.
Then, sadly, you meet the world - and you make your way thru childhood, teens, and finally into full blown life.
That's when the skin begins to thicken.
Seems to me like it is not just a part of the aging process. A thick skin is more of a defense mechanism against the mental corruption, mindless rat races, and endless shallowness that we get exposed to more and more as we go thru life.