The reason that the Roth IRA custodian asks these questions is to determine what code to put on the Form 1099-R. In this case you would answer would be "Premature (under 59½ years old)" so that the Form 1099-R will have a code J in box 7. (It's a bit odd that the custodian would be asking you to provide this information since they should have a record of your birth date from when you opened the IRA.)
The taxable amount is determined on your tax return. TurboTax will prepare Form 8606 Part III where it will subtract your contribution basis from the distribution amount and, if the amount distributed is less than your contribution basis, the taxable amount will be zero.
