Ok ... so you married the mother ? And you are not the biological father ?
If you are married then why did you not file a joint return with the mother? Did you live together ?
Did you try to claim Head of Household using this child ?
And for a child under the age of 19 support is not an issue ... the child themselves cannot be self supporting. The IRS wants you to prove they are your kids to claim and that they lived with you for more than 1/2 the year.
