Squirrels can be difficult to age and many observers simply group them into juveniles (up to six months old), subadults (six months to one year) and adults (one year plus) based on the colour of the fur around the genitals, swollen mammae, fur pattern, density and pigmentation of the fur. Juvenile animals have sparser fur than adults and the tail hair is short and dense. Body weight can also be an accurate separator.
In the laboratory, age can be determined by sectioning the teeth; a broad opaque layer of cementum is deposited in the roots of the molars during the summer, with a narrow dark band forming during the winter.
In the wild, Red squirrels are known to live for up to seven years, although three appears more typical. The oldest captive Red squirrel that I am aware of was a male that lived to 15 years old at Asahiyama Zoo in Japan.
Grey squirrels can live for up to nine years in the wild, four or five being fairly typical, and the oldest captive specimen on record was an animal of unknown sex at Wisconsin's Racine Zoo which survived to 23 years and 6 months old. Studies on Grey squirrel populations in Southern England during the early 1980s by John Gurnell, revealed a difference in survival between the sexes, with females living—on average—longer than males (4 to 5 and up to 7 years, versus 2 to 3 years and up to 5 years).