You cannot choose to use Form 1040 in TurboTax Online. TurboTax automatically uses Form 1040-SR if you are 65 or older. In the CD/Download TurboTax software there is a box you can check in forms mode to print Form 1040 instead of Form 1040-SR. But TurboTax Online does not have forms mode, so there is no way to check the box.
