This TurboTax FAQ link leads you to: How to Claim the Disability Amount Retroactively
If needed, this TurboTax FAQ link holds a variety of facts to be reviewed: A Guide to the Disability Tax Credit
Claiming the DTC for Previous Years is part of the information found within the above FAQ.
If the CRA determines you should have been eligible for the DTC in previous tax years, you can file for this credit retroactively. In order to do so, file Form T1-ADJ, either via mail or using your CRA My Account, for every return you need amended, for up to ten years past. The CRA will review your adjustment requests and send you any refunds you may be owed.
More information can be found on the Canada Revenue Agency site here: Disability tax credit (DTC)