The age deduction is calculated automatically if you eligible for it. You are eligible for this amount if you were 65 years of age or older on December 31, 2017, and your net income is less than $84,597. If you were an immigrant or emigrant, your claim is prorated for the time you were resident in Canada.

The age amount is a non-refundable tax credit. So if your tax payable is already at Zero, you don’t get any of the credit.

https://turbotax.intuit.ca/tips/claiming-the-age-amount-tax-deduction-5568