Tattoos do not wear off because the tattoo pigment particles are embedded in the dermis, and the body is unequipped to remove particles of a certain size from the area. The body naturally turns over skin, but this process does not affect particles in the dermis.
The upper layer of the skin is called the epidermis and continually regenerates by the natural process of skin turnover. One complete cycle typically takes around six to eight weeks. When getting a tattoo, the pigment is injected under the epidermis in the dermal layer. Skin in the dermis is not subject to the same process of skin turnover.
Ordinarily, the body has cells whose primary function is to transport foreign material or particles by engulfing them and bringing them to the lymph glands. While the body initially identifies the tattoo pigments as foreign material, the cells are unable to remove the pigment particles due to their size. As a result, the body surrounds the pigment particles at the microscopic level with a thin layer of scar or fibrous tissue. The tattoo pigments eventually become permanently trapped in the dermis. While a tattoo may fade to a degree over a long period of time, it generally remains permanent.