Why is my son not eligible for a dependent deduction when he turned 18 late in 2014?

The child must have been 17 years of age or younger in order to claim them unless they have an impairment in physical or mental functions.  The claim is not prorated in the year the child turns 18.  Canada Revenue says : 

You can claim $2,255 for each of your or your spouse's or common-law partner's children who are under 18 years of age at the end of the year.

You may be eligible to claim the family caregiver amount for each child with an impairment in physical or mental functions.

The full amount can be claimed in the year of the child's birth, death, or adoption.

That is correct.  The Amount for an eligible dependant (formerly known as the Equivalent to married exemption) can be claimed under certain situations ie) a single parent;  and if at any time in the year you supported your child under the age of 18.  As your child was under the age of 18 during the year, and assuming you meet all of the conditions, you would be entitled to this amount for the entire year.  It is not prorated .  For more info and to see if you are eligible to claim this amount, please see http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/305/menu-eng.html 

Thanks for that. What about an equivalent to spouse deduction? Do the same rules apply? I've heard that as long as the child was under 18 most of the year it does apply. Turbo Tax doesn't seem to make allowance for it.