Yes, being age 18 (or even 17) does not prohibit you from claiming Head of Household (HoH) filing status and claiming your younger siblings as dependents.
The main requirements for Head of Household are: a taxpayer that
1) is single or did not live with their spouse at any time during the last
half of the tax year.
2) has at least one closely related dependent (usually a child) that lived
with the taxpayer for more than half the year
3) pays over half of the support of the household expenses, including rent
/ mortgage... It is not necessary that you own the house, only that you paid more than half the expenses.
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.A child closely related (siblings count) to a taxpayer can be a
“Qualifying Child (QC)” dependent, regardless of the child's income, if:
1.
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
2.
He did not provide more than 1/2 his own
support
3.
He lived with the relative (including temporary
absences) for more than half the year
4.
He is younger than the relative (not applicable
for a disabled child)
5.
If the
child meets the rules to be a qualifying child of more than one person, you
must be the person entitled to claim the child as a qualifying child (this
essentially means that you have the parent’s permission to claim the child, if the child also lived with the
parent more than half the year)
6.
If the parents of a
child can claim the child as a qualifying child but no parent so claims the
child, no one else can claim the child as a qualifying child unless that
person's adjusted gross income (AGI) is higher than the highest AGI of any of
the child's parents who can claim the child.
