It generally asked your household income. This means many 18 year olds can report 6 or even 7 figure household incomes.
Do you have a source for this? Or can you define "many" some more?
(Slightly over a quarter of households in the US have a household income over $100K as reported on tax returns. Household income for the purposes of credit application can be meaningfully higher than for adjusted gross income tax return concerns.)
it's 26.3% of households with incomes over $100K. I saw another estimate of 125 million households, so it could well be ~33 million households with >=$100K household income.
(I'm not so much quibbling with numbers as trying to get across that "it's a lot of people")
Even after you're 21 you're supposed to only include someone else's income if you have access to it. If your parents don't give you a stipend then you aren't supposed to include their income.