The age deduction (age amount) 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 (line 236 of your return) is less than $84,597. If you were an immigrant or emigrant, your claim is prorated for the time you were resident in Canada.

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

I am over 77 years old and have been ion Canada since 1965 but the age amount calculated is $6972.57

The maximum age amount if you are born prior to 1954, is $7494 depending on your total income.

Please review this link from Canada Revenue Agency for more detailed information.