How old do you have to be to use an automated external defibrillator?

Anyone can use it. Anyone that can follow the instructions can use it, even high school children.
Almost anyone. Almost anyone who can understand and follow instructions. Generally the american heart association, aha, recommends that children are at least 8 years old before they take the training for basic life saving, bls, or cpr. This training includes when to and how to use an automated external defibrillator or aed. Additionally, the machine gives instructions while in use.