How to 'spot' a giraffe's age?

The result of several decades of research has indicated that the age of a giraffe can be estimated by looking at the animal's spots.

The relationship between spots and age was as the result of a series of observations recorded by a park ranger in Zambia. The ranger began recording his observations in the early 1970s. The observations have been reviewed by Fred Bercovitch, a biologist based at the Primate Research Institute and Wildlife Research Centre at Kyoto University, Japan.
The study was undertaken by following male Thornicroft’s giraffes (or Rhodesian giraffe) in Zambia, Africa. Giraffes, at almost 19 feet, are the tallest of all mammals. Adult males weigh as much as 4,000 lbs. They are common to the Luanga Valley, northern Zambia.
The study was published in the Journal of Zoology.
The study revealed that the color of a giraffe’s brown patches can be used to estimate the mammal's age. Specifically the coats of male giraffe's darkened around seven years of age, transforming from brown to black over a two year period.
The BBC quotes Bercovitch as saying that the change in color is probably due to rising testosterone levels and act as a signal puberty to other giraffes. Bercovitch goes onto say "The main contribution of our work is that we could attach specific ages to coat color changes, which provides something of a ‘biomarker’ of aging in giraffe.”
The study also revealed that male Thornicroft’s giraffes have a maximum lifespan of twenty-two years in the wild. The research has also shown that giraffes tend to associate with friends and relatives.