If you are strictly talking about adding a dependent - then follow the information below.
However, if you are really meaning that your child is
not giving you the Child Tax Credit. This is because once a child turns 17 they are
no longer eligible for the Child Tax Credit. This is a separate credit from being claimed as a dependent, but it is a valuable credit because it can have a refundable portion of up to $1,000. But, again, if your child was 17 for even one day in 2017, then they will not be eligible for this credit anymore. See
Child Tax Credit
If you need to add dependent -
If you haven't filed already, then you can simply add them as a dependent when working through TurboTax. If you have already gone through the My Info section once, then you will have to return to this section to add a dependent. Click My Info and this will bring you to the Personal Info Summary Screen. Click the "Add Dependent" option and the next screen you will get will allow you to begin the process of adding a dependent. See screenshots below.
If you have already filed, then you will need to amend your return to include another dependent. Click
here for instructions on how to file your amended return. After you start the process, you can follow the same steps as above to add a dependent.
Note: When adding a dependent, be sure to read the screens carefully. There is one that sometimes causes confusion and it refers to whether your child provided more than half their own support. This question is not asking if you provided more than half their support - it's asking if the child provided more than half of their own support (likely no). So, be sure to answer it correctly.
