Can a person age 19 be claimed as an exemption on federal tax forms as an IRS dependent?

Yes if and when all of the necessary rules have been met by each taxpayer that is named on the 1040 income tax return. Not as a IRS dependent but as a dependent on the qualifying taxpayers income tax return that qualifies to claim the 19 as an dependent for the exemption amount on the 1040 income tax form.

For all of the rules go to the IRS gov website and use the search box for PUBLICATION 17 go to chapter 3. Personal Exemptions and Dependents

Each test is explained in chapter 3 of the publication 17 and you will also find the worksheet that you can use to determine if the support test is met.