Yes, you can claim an older car. You'll have to make a reasonable estimate on the Fair Market Value (FMV) of your vehicle on the date you started using it for Business or Employment use (2017). One way you could get a fairly accurate calculation would be to use the actual purchase price - minus - the amount you would have claimed for CCA if you had used it starting on the purchase date. Use this figure as the cost of your "addition". If you were questioned by CRA as to how you came up with this figure, you would be able to show them you used this method of calculation and considered it to be reasonable.
If I started using the vehicle from the date it was purchased, but never claimed the CCA, can I claim the CCA on the original purchase price or is it too late?
If you want, you can go back to the tax year when you started using the vehicle for business and make an adjustment to add the vehicle (as well as adjusting any returns in between where you want to claim CCA). But if you don't want to do this, then you'll need to add the vehicle this year at the Fair Market Value (FMV) or use the actual purchase price minus the amount you would have claimed for CCA if you had used it starting on the purchase date.