T mobile asked me for my ssn?

Today i saw that you can get the iphone 5 without a contract on t mobile if you pay a $99 down payment my question is. Why did t mobile asked for a ssn? Do i need to be 18 years old?

T-Mobile requires a credit check if you plan on getting any new phone and making monthly installments on it. T-Mobile is considered a prepaid service without any credit checks only if you plan on making monthly installments to pay off a new phone.