If you file a tax return for any reason, you must report all your income, no matter how small. There is no investment minimum.
You may be thinking of the "kiddie tax." Most taxpayers have a standard deduction of $12,550 (this year). If a person's taxable income is less than the standard deduction, they don't have to file a return, because no tax is owed if their income is less than the standard deduction.
However, for dependents and people who meet certain dependent-like tests (under age 19, or under age 24 while a student), their standard deduction is variable. If they have investment income or other unearned income, their standard deduction is $2,200. If they have a mix of income earned from work and unearned income, their standard deduction is determined by a formula.
https://www.irs.gov/taxtopics/tc553
If taxable income is less than the standard amount, no tax return is owed. If more than the adjusted standard deduction, they must file a tax return and may be taxed at a higher rate than normal. (The purpose is to prevent parents from hiding investment income in a child's name and pay lower tax.)
The short answer is that a person who is a full time student under age 24, who does not work, can have up to $2200 of unearned income such as investment sales, without having to file a tax return. The $2200 threshold is per year, not lifetime aggregate.
