If you were 19 at the end of 2019 and not a full-time student then you cannot be claimed as a dependent.
Simple answer: No.
But taxes aren't simple; so "it depends".
When did you go on active duty? Did you live with you mom before that? Were you in school (including high school) earlier in the year? When did you complete training (basic & AIT)? What was your income for the year?
A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:
- He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
- He did not provide more than 1/2 his own support. Scholarships are considered third party support and not as support provided by the student.
- He lived with the parent (including temporary absences such as military training) for more than half the year
So, it doesn't matter how much he earned. What matters is how much he spent on support. Money he put into savings does not count as support he spent on him self.
The support value of the home, provided by the parent, is the fair market rental value of the home plus utilities & other expenses divided by the number of occupants.
So, if all three of those rules are not met and your income for the year was more than $4200, you cannot be her dependent. Note that the income rule only comes into play, if the other 3 rules are not met.