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Yes a child that is 18 and earned some income can be claimed so long as you provided more than half the support of the child.
For details on all the qualifications see A "Qualifying Child" :
In general, to be a taxpayer's qualifying child, a person must satisfy four tests:
Temporary absence from home to attend college is still counted as time the child resided with you at home.
I hope this helps for explaining the qualifications to claim a dependent when the child had some earnings.