This is what we found on Wikipedia regarding the Foxing:

Foxing is an age-related process of deterioration that causes spots and browning on old paper documents such as books, postage stamps, old paper money and certificates. The name may derive from the fox-like reddish-brown color of the stains, caused by the rust chemical ferric oxide which may be involved. Paper so affected is said to be "foxed".

Foxing is rarely found in incunabula, or books printed before 1501. Decrease in rag fibre quality may be a culprit; as demand for paper rose in later centuries, papermakers used less water and spent less time cleansing the rag fibres used to make paper. An early work of art to have been affected by foxing is the Portrait of a Man in Red Chalk, a drawing on paper by Leonardo da Vinci.

Foxing also occurs in biological study skins or specimens, as an effect of chemical reactions or mold on melanin.

Foxing can also be intentionally added to clothing to give it an aged appearance.

Aside from foxing, other types of age-related paper deterioration include destruction of the lignin by sunlight and absorbed atmospheric pollution, typically causing the paper to become brown and crumble at the edges, and acid-related damage to cheap paper such as newsprint, which is manufactured without neutralizing acidic contaminants.