Why or where does turbo Tax allow this entry? S.C. income tax deduction of up to $15,000 against any taxable income of a individual 65 or older in tax year. 

[LSMcEntire]

South Carolina Code §12-6-1170(B) provides an income tax deduction of up to $15,000 against any South Carolina taxable income of a resident individual who is 65 or older by the end of the tax year. The following requirements apply to this deduction:  Amounts deducted as retirement income under South Carolina Code §12-6-1170(A) reduce this $15,000 deduction.  Amounts deducted as a surviving spouse under South Carolina Code §12-6-1170(A) do not reduce this $15,000 deduction.  Taxpayers’ filing a joint return are allowed a deduction of up to $15,000 when only one spouse is 65 or older, and up to $30,000 when both spouses are 65 or older, by the end of the tax year.

[BillM223]

The up-to-$15,000 retirement deduction in South Carolina Code §12-6-1170(B) appears on line 3q on form SC1040.

In the input (the screenshot is from the CD/download product - Online may be different), you will see this screen:

If you think the size of the deduction should be different, correct it here.