Step 1 : Introduction to the question "How often does human skin replace itself with a new layer?"
The external, or top, layer of human skin is the epidermis. This layer sheds itself, one skin cell at a time, through the process of exfoliation, completely replacing the old layer with a new one about every 27 days. The deeper layers of skin tissue beneath the epidermis don't participate in exfoliation. Scars usually form in the deeper layers of the skin and because these layers are not replaced, the scars remain. Tattoo ink will typically affect the epidermis and the layer beneath (the dermis), and thus does not disappear with the body's natural shedding of the outer skin layer. Source: Reference.com