Why is my iPhone 4 suddenly asking for payment information when I try to download a free song?

I have an iPhone 4 and I have never once had an issue with the payment thingie. When I made my Apple ID, it didn't require me to add a credit card number. I don't even own a credit card, I'm only 16 years old. My parents don't wanna put theirs on my account. After changing my password, it keeps asking for payment information when I try to buy FREE songs (the free singles of the week) or free apps. Any information would be helpful, please and thank you.

FWI, I have iOS 7 but I'm not sure which version; I have the first one that came out, I haven't updated it since then because it makes my iPhone run slowly sometimes.