Iguanas live a really long time
People have a tendency to acquire iguanas without fully understanding their basic needs for a healthy life. This includes a warm environment (75 to 95 degrees Fahrenheit), access to UVB light, a properly sized habitat and a healthy diet of vegetables.
Depending on the species, iguanas can live anywhere from four to 60 years. This potentially long lifespan is exactly why purchasing one of these creatures on a whim is a terrible idea.
If you're interested in taking on an iguana as a pet, the IguanaPet site has an illuminating list of myths and misconceptions about proper iguana care that both current and prospective owners should read.