My son is 21 and daughter is 17. I have declared them on my tax file. Does my son have to produce a tax file even if he is declared on mine?

Though you have declared your son on your return he should file his own return. There are multiple benefits to doing so such as the Education Tax Credit, Textbook credit, GST/HST credit, and Extra RRSP contribution room. 

Here is a link to an article by the Canada Revenue Agency on the benefits of why he should file his tax return.

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