We'll be getting passports for our 2 DSs, in the spring. We're waiting until April because our older DS will turn 16 at the end of March, and at the age of 16, the passport is good for 10 years. Under the age of 16 is only good for 5 years.
I know some people consider passports to be a waste of money, especially for children, but we just feel better traveling with them, since that's all you need for identification.