I was reassessed by CRA for the age deduction. It's a calculated amount, so how can that happen?

Can you provide us with some more details, please? 

1. Which tax year did you file that you were reassessed on? 
2. Which version of TurboTax did you use to prepare the tax return? Was it installed in your computer, or was it the OnLine version?
3. Did you turn 65 in the year you were reassessed? 
4. Does CRA have your correct Birthdate on file?

A reassessment on the "Age Amount" is almost unheard of as you mentioned, "it's a calculated field based on your entries". If you had mistakenly entered an incorrect Date of Birth, you would have probably been held up at NetFile. 

If this is a TurboTax calculation error, we will stand behind it with 100% Accuracy Guarantee. Look forward to helping you solve this issue. 

As we do guarantee our calculations to be 100% accurate, I'm attaching the instructions below for you to submit a claim under our 100% Accurate Calculations Guarantee. Thanks for being a loyal TurboTax customer!! 

https://turbotax.community.intuit.ca/replies/2647678

It wasn't a large amount. Contacting CRA to ask why amounts were changed on reassessment. 

2017 tax year

Vers. 00.01.81 or most recent update

On computer

Yes, I turned 65

Yes, CRA have correct birthdate