Start Saving Sooner Act of 2015

Amends the Internal Revenue Code to establish a tax-exempt individual retirement account for taxpayers under age 18, to be known as a young savers account. Treats such accounts as Roth individual retirement accounts for income tax purposes. Allows the tax credit for retirement savings for contributions to a young savers account. Directs the Secretary of the Treasury to pay any overpayment of tax that is attributable to the tax credit for retirement savings to the taxpayer's young savers account.