Sorry, this is not a glitch in the program. CRA requirements are that the dependant for whom you are claiming the Eligible Dependant Tax Credit be "under 18". Although it does seem you should be eligible to claim this because your son was "under 18" at some point in the year, CRA uses the age on December 31st of the tax year.
CRA does not clearly state this, however there is a slight reference to the under 18 years of age at the end of the tax year rule in this CRA documentation about the Eligible Dependant Amount: https://bit.ly/2U4mL6A
