Young lobsters start off at just 2.5 cm and adults are typically about 10 cm from the tip of the antennae to the tail. It takes about 7 years to get to this size. Very few make it to 20 years old but these may reach 45 cm and weigh 4.5 kg.Because they have hard shell (exoskeletons) spiny lobsters have to shed them to grow. During this molt the lobster bulks up with water to expand the new exoskeleton. This then firms up and becomes hard within a week. Spiny lobsters molt about 4 times per year.