See iTunes TOU here. http://www.apple.com/legal/itunes/us/terms.html#GIFTS
"This Service is available for individuals aged 13 years or older. If you are 13 or older but under the age of 18, you should review this Agreement with your parent or guardian to make sure that you and your parent or guardian understand it."
She's perfunctory that way.
Is the app a free app or one you need to buy?
What is happening when you try to get the app? Can you describe it? Is there any instruction for a next step to resolve the issue?
Did you create the account for yourself through the Mac App Store or through the iTunes Store? Have you logged into the account through the Mac App Store and inspected the account information? If there is a bank card, is all the required information filled in? Have you logged into the account through the iTunes Store and inspected the account information? Is everything filled in correctly?