Development of a yellow cast in aging paint, most noticeable in the dried films of white paints or clear varnishes.
Alkyd/oil based paints, because of their curing mechanism, tend to yellow, particularly in areas that are not exposed to sunlight.
Oil-based varnishes start with an amber cast and will darken with age.
Heat from stoves, radiators, and heating ducts.
Lack of light, for example, behind pictures or appliances and inside closets.
Tobacco staining or other environmental contaminants.
If there are no other problems and the yellowing is not offensive, repainting is not necessary.
Repainting using a latex paint will reduce the amount of yellowing, but if the environmental conditions that caused the previous coating to yellow continue, any new coating will likely yellow as well.