You would need to either refer to the retirement plan agreement or ask the plan administrator what the minimum retirement age is for the plan. TurboTax asks the question because disability distributions (code 3 in box 7) received before the minimum retirement age are treated as wages and are therefore treated slightly differently on the tax return.
Yes, these would be periodic payments. TurboTax asks this as part of the information necessary to determine if one might need to use the Simplified Method for determining the taxable amount, but if the amount shown in box 2a of the Form 1099-R is correct, one should not be using the Simplified Method to calculate anything different.
