From Wikipedia -
Like other spiders, tarantulas have to shed their exoskeleton periodically in order to grow, a process called moulting. A young tarantula may do this several times a year as a part of the maturation process, while full grown specimens will only moult once a year or less, or sooner in order to replace lost limbs or lost urticating hairs. A tarantula is obviously going to moult (or "shed", as some call it) when the exoskeleton takes on a darker shade. If a tarantula previously used its urticating hairs, the bald patch will turn from a peach color to deep blue.

Tarantulas may live for years—most species taking 2 to 5 years to reach adulthood, but some species may take up to 10 years to reach full maturity. Upon reaching adulthood, males typically have but a 1 to 1.5 year period left to live and will immediately go in search of a female with which to mate. Male tarantulas rarely moult again once they reach adulthood. The oldest spider, according to Guinness World Records, lived to be 49 years old.