What country are you in ? In the US and here in the UK the minimum age is 13, not sure whether other countries are different.
If you only want to download free items from your country's store then there are instructions on this page for how to create a new account without giving payment details : http://support.apple.com/kb/HT2534
e.g. if currently logged into an account then log out of it, find a free app in the store and click on its free 'price', and then click on 'create Apple id' on the popup that you get