For adult dependents to qualify, the person claiming them on their tax return must meet the same income threshold as everyone else: $1,400 payments for individuals with an adjusted gross income of up to $75,000, heads of household with income up to $112,500, and $150,000 for joint filers.
After that threshold, the payments will be reduced until a max income limit of: $80,000 for single filers, $120,000 for heads of household, and $160,000 for married couples. No one with income above those caps will receive money in this round of stimulus payments.
Your income is determined by your most recent tax return — 2019 if the IRS has not yet processed your 2020 return.
Adult dependents do qualify for stimulus money. In order to get the 1st and 2nd stimulus for your dependent, you must include dependent on your 2020 tax return and the stimulus checks will be issued as a recovery rebate credit. Your dependent also qualifies for the third stimulus. Please see attached article which addresses the Third Economic Impact Payment: https://www.irs.gov/coronavirus/third-economic-impact-payment