Thank you for your question,
If you want to claim an eligible tax credit for your older child, you should have some criteria.
As per the Canada Revenue Agency (CRA):
if you have not claimed an amount for the year on line 30300 of your return, you may be able to claim this amount for one dependant if, at any time in the year, you met all the following conditions at once:
- You did not have a spouse or common-law partner or, if you did, you were not living with, supporting, or being supported by that person
- You supported the dependant in 2020
- You lived with the dependant (in most cases in Canada) in a home you maintained. You cannot claim this amount for a person who was only visiting you
In addition, at the time you met the above conditions, the dependant must also have been either:
- your parent or grandparent by blood, marriage, common-law partnership, or adoption
- your child, grandchild, brother, or sister by blood, marriage, common-law partnership, or adoption and was under 18 years of age or had an impairment in physical or mental functions
Because your child is 18 years old and also you live with your common-law partner, you cannot claim an eligible dependant tax credit for your older child.
You can get more information on the website of Claiming the Amount for an Eligible Dependant Credit
However, you may claim Canada caregiver tax credit for an older child. For getting Canada caregiver tax credit, your child depends on you for support because of physical or mental impairment. Because your mutual child doesn't have any physical or mental impairment, you cannot claim Canada Caregiver tax credit for him. You may claim Canada Caregiver tax credit for your older son because he has a mental disability.
You can get more information on the website of What amount can you claim?
