How old do you have to be to get a overdraft?

If you're referring to a bank overdraft, any age really. An overdraft is just when you've spent more than you have in your account. Different banks have different policies. Some allow you to tie your (presumably) checking account to a savings account, and provided that you have funds in the savings account, they will automatically move money over for you (for a fee of course). If you are over 18 and eligible, you can also sign up for a line of credit which is basically a credit card, only with out the card and only for your bank account. If you are out of money in checking and savings, and you have a line of credit, they will take money from there. Many places don't like having to charge you for poor money management, but they do it anyways because it is a potential avenue to make some money. If you find yourself in a hard spot, talk to your bank because many times they will forgive your first overdraft.