Unfortunately, ageing hedgehogs is not a simple task. Eye lenses are known to increase their dry weight progressively and can provide a useful index of relative age, but this technique requires a freshly dead animal. In Hedgehogs, Nigel Reeve notes that low-power dental X-rays can be used to assess the degree of epiphyseal fusion of the forelimbs, this is where cartilage at the end of long bones ossifies, but this is complete by 18 months, making the technique useful only for youngsters.
Currently, the most reliable technique involves ventrobuccal (lower jaw) sectioning to count the periosteal growth rings. Changes to calcium metabolism during hibernation results in a slowing of bone deposition in the periosteum, the thin connective tissue membrane covering the bones. When the bone is cross-sectioned and stained rings can be seen. The rings vary in size but work in much the same way as tree rings; a ring represents one year. Even this method is not 100% accurate because physiological stress such as pregnancy or illness can cause ring formation; changes in climate that affect hibernation duration can also alter ring development. Perhaps most importantly, the technique cannot be performed on a live animal.
In some instances, it may be possible to estimate an animal’s age based on obvious morphological features, including claws and teeth. Both wear down throughout the animal’s lifetime so strong pointed claws and sharp, barely worn, teeth usually signify a yearling, while old age is often indicated by cheek teeth worn and covered with tartar. Unfortunately, these have their limitations. Animals feeding in gritty soils will experience a more rapid wearing of teeth than those foraging in more peaty habitats, for example. Old individuals often have a 'gingery' tint to their fur, but this is of limited use as an aging tool.
Based on analysis of 83 road-kill hedgehogs, a team led by Amy Haigh at University College Cork in Ireland found that adults had a hind foot length greater than 36mm (1.4 in.), a body length of more than 160mm (6.3 in.), a jaw length exceeding 45mm (1.8 in.) and fully-grown spines; animals with measurements below this and with growing spines were considered juveniles, less than a year old.
The average age for a wild hedgehog is widely cited as five or six years, although the reality is probably closer to three or four years. The upper age limit for this species in the wild appears to be about nine years old. Amy Haigh and her colleagues had two males aged eight and two females aged nine in their sample. In his compendium on the longevity of mammals in captivity, Richard Weigl gives the oldest captive hedgehog as a male born in Cleveland Metroparks Zoo in 1984 that died in September 1995 at 11 years and eight months old. In a 2005 paper in the German anatomical journal Kleintierpraxis, Wilfried Meyer and Klaus Pohlmeyer at the Anatomisches Institut in Hanover counted 15 rings in the jaw of an “old female” hedgehog kept in captivity; a sixteenth ring was apparently ambiguous. This post mortem suggests that hedgehogs have the potential to reach at least 15 years of age and, to the best of my knowledge, represents the oldest verifiable age for a European hedgehog on record.