Aging is a process by which an artwork, typically a painting or sculpture, is made to appear old.
[1] Paintings deteriorate over time because they are created using essentially incompatible materials, with each having a different reaction to the changes in the environment, including light, temperature and relative humidity.
When this occurs, expansion and contraction of the painting will result in a crazing of the varnish surface.
Along with the darkening or yellowing of the varnish surface, it is this visual representation of the cracking that is typically the primary indicator of ageing.
The purpose for artificially ageing is to create a finished product that accurately reflects an era or is consistent with the environment (usually period) into which it is to be placed.