The rule is still the same for the Premium Tax Credit: Household Income includes the dependent's income *IF* the dependent has "taxable income" on their tax return (income after the Standard Deduction or Itemized deductions).
If the dependent only has earned income (such as from a job or self employment), then yes, the dependent can earn up to $12,000 of income in 2018 before it would be included. However, if the dependent has more than $350 of "unearned" income (interest, dividends, capital gains, taxable scholarships, unemployment, etc.), then that could trigger the requirement to include the dependent's income (because the Standard Deduction for a dependent with unearned income is lower).
