If they have earned income from work and received a W-2 then they should file if taxes were withheld from their income. If no taxes were withheld on the W-2 or their earned income was less then $12,950 they do not have to file
if they have earned income from self-employment and have $400 of income then they are required to file a tax return reporting their self-employment income and expenses on Schedule C.
If they have unearned income from interest, dividends or capital gains of more than $1,150 they are required to file a tax return.
