The $400 in this question has a long history. In 1950, Congress amended the Social Security Act to bring most self-employed workers outside farming into the old-age and survivors insurance system, with exceptions for some professional groups. The change took effect on January 1, 1951, and a self-employed person was covered when annual net income from self-employment was at least $400, according to the Social Security Administration's summary of the 1950 amendments.
Later laws widened the circle. The agency's coverage history lists professional groups added in 1954 and 1956, and self-employed physicians in 1965. The same $400 shows up today in the Internal Revenue Service (IRS) instructions for Schedule SE. Schedule SE is the form where a self-employed person figures self-employment tax, and the instructions say a self-employed person must pay it with net earnings of $400 or more.
So the $400 line comes from Social Security law, not from the income tax brackets. It applies to a fifteen-year-old mowing lawns just as it applies to an adult consultant. And it is only one of three separate tests that decide whether a teenager's summer money leads to a tax return.
A summer that adds up faster than expected
Take Maya, 16, who spent summer 2026 mowing lawns for neighbors and watching a toddler two afternoons a week. By September she had been paid $3,000, mostly in cash and by phone app. One neighbor who runs a landscaping company paid her $2,100 and later mailed a Form 1099-NEC (the document where a client tells the IRS how much it paid a contractor). She spent $200 on gas and a pair of shears. (All numbers in this example are invented for illustration.)
Her parents know that the standard deduction, the amount the IRS subtracts from income before figuring tax, is thousands of dollars higher than $3,000, and they assume that settles it. It settles one test. The other two work differently.
Test one: the income tax limits for a dependent
A dependent is a person that another taxpayer can claim on their return. For most teenagers that means a parent. The IRS sets lower, special filing limits for dependents, listed in Publication 501 (the IRS guide to dependents, the standard deduction and filing requirements), and they depend on what kind of money came in.
Earned income is pay for work: wages, tips, fees. Unearned income is money from interest, dividends or investment gains, such as a savings account a grandparent opened. For 2025, Publication 501 says a single dependent under 65 who is not blind must file a federal return if any of these is true:
- unearned income was more than $1,350;
- earned income was more than $15,750;
- total (gross) income was more than the larger of $1,350, or earned income up to $15,300 plus $450.
These limits change every year. For 2026, the IRS has set the standard deduction for a single person at $16,100, in its annual document of inflation adjustments. For someone who can be claimed as a dependent, it cannot exceed the greater of $1,350 or $450 plus the person's earned income. In plain terms, the limits are set so that a dependent with modest earned income does not have to file, and Maya's $3,000 is far below them.
A return can still pay off when no tax is owed. The IRS's filing guide says a person may want to file even when not required, to get back tax that an employer withheld from pay. Say a teenager worked at a summer camp for $4,000 on a regular paycheck, with $120 of federal income tax withheld. That teenager is under the dependent limits and has no filing requirement from this test. Filing would still be the way to ask for the $120 back. The same guide notes that a refundable tax credit (a credit paid out even when no tax is owed) can also be a reason to file, which depends on the person's own facts.
Test two: the self-employment rule
The second test ignores the dependent limits. Publication 501 also contains Table 3, titled "Other Situations When You Must File a Return." It is a list of cases in which a return is required no matter how much income there was, and it covers dependents too. It includes, for example, special taxes, distributions from health savings accounts (HSAs) and net earnings from self-employment. That last item is the one that applies to Maya: net earnings from self-employment of at least $400.
Self-employment means working for yourself. The Schedule SE instructions put it in one line: if a person is in business for themselves, they are self-employed. Mowing lawns for neighbors, tutoring or walking dogs can fit that description when the person works for themselves rather than for an employer, even if the work is small and the customers pay in cash. A person like this reports the income and expenses on Schedule C and figures self-employment tax on Schedule SE, both listed on the IRS's gig work page.
Self-employment tax is how self-employed people pay into Social Security and Medicare. It is similar to the Social Security and Medicare taxes withheld from most employees' pay, except that nobody withholds it for a self-employed person. The IRS rate is 15.3%, made up of 12.4% for Social Security and 2.9% for Medicare. It is charged on 92.35% of net earnings, according to IRS Topic 554, where net earnings means income from the work minus ordinary and necessary business expenses.
The exact threshold is easy to misread. IRS web pages describe the trigger as net earnings of $400, and the IRS guide "Check if you need to file a tax return" even says "over $400." The Schedule SE instructions put it in dollars of business profit: file Schedule SE if profit is $434 or more. The two agree after the 92.35% step, because $434 × 0.9235 is about $400.80, while $433 comes to $399.88. In practice, a profit of $434 crosses the line.
For Maya, using the invented numbers:
- Profit: $3,000 − $200 = $2,800.
- Amount subject to the tax: $2,800 × 0.9235 = $2,585.80.
- Self-employment tax: $2,585.80 × 0.153 = about $395.63.
Her profit is far above $434, so under the IRS rules a federal return is needed in this example, even though her income tax limit was never reached. The $396 or so is a Social Security and Medicare contribution, separate from income tax. The example ignores other items on the return, such as the deduction for half of the self-employment tax.
The threshold applies to profit, not to the total paid, and expenses lower the profit. For example, a teenager paid $500 who spent $100 on supplies has a $400 profit, under the $434 line. A teenager paid the same $500 with only $30 of costs has a $470 profit, over it. The first stays under the self-employment trigger and the second crosses it, even though the two paychecks looked alike. Only costs that are ordinary and necessary for the work count, so receipts matter when expenses are claimed.
One more detail trips families up. The Form 1099-NEC the landscaper sent is a report to the IRS, and a business must send one when it pays $600 for services, or $2,000 for payments made after December 31, 2025 (see the IRS 1099-NEC guidance). That rule only decides who sends a form. As the IRS gig work page puts it, all income must be reported "even if you don't receive Forms 1099," so Maya's $900 of other jobs counts as much as the $2,100 that came with a form.
Test three: California
A California resident faces a separate state test. It is described in the 2025 Form 540 booklet from the Franchise Tax Board, California's tax agency. Form 540 is the state income tax return for residents. The booklet says a person who can be claimed as a dependent has a filing requirement when income is more than their own California standard deduction. That deduction is figured on a worksheet, has a minimum of $1,350, and for a single filer is capped at $5,706 for 2025.
The cap matters because it is much lower than the federal limit. Say a teenager earned $6,000 in wages during 2025. That is under the federal earned-income limit of $15,750 and over California's $5,706 cap. One state return could be required where no federal return is. The 2026 figures may differ, so the current booklet is the place to check.
What the question comes down to
Back to Maya. For the same summer, the three tests give her three different answers. On income tax, her $3,000 is far below the dependent limit, and if all her work had been as an employee, that would have settled it. But she worked for herself, so the self-employment rule applies: a profit of $2,800 is well above $434, and under the IRS rules she needs a federal return with about $396 of self-employment tax. For California, the answer depends on the 2026 figure, which should be checked against the current booklet.
Whether a return is needed depends on how the money was earned, as an employee or for yourself, as much as on how much of it there is. For the self-employed, what counts is the profit after ordinary expenses, so two teenagers with the same payment can land on opposite sides of the line. And owing no tax does not mean no return is needed: when an employer withheld tax from pay, a return can be worth filing even if none is required.
This is one illustrative case. A real family has its own circumstances, such as other income, withheld tax and tax credits, and those are easier to sort out with a tax professional.
Sources
- Social Security Administration: Social Security Act Amendments of 1950 in brief.
https://ssa.gov/policy/docs/ssb/v13n9/v13n9p3.pdf - Social Security Administration: history of OASDI coverage, financing and insured status.
https://www.ssa.gov/policy/docs/statcomps/supplement/2018/2a1-2a7.html - IRS: Instructions for Schedule SE (Form 1040).
https://www.irs.gov/instructions/i1040sse - IRS: Publication 501, Dependents, Standard Deduction, and Filing Information.
https://www.irs.gov/publications/p501 - IRS: Revenue Procedure 2025-32 (2026 inflation adjustments).
https://www.irs.gov/pub/irs-drop/rp-25-32.pdf - IRS: Check if you need to file a tax return.
https://www.irs.gov/individuals/check-if-you-need-to-file-a-tax-return - IRS: Manage taxes for your gig work.
https://www.irs.gov/businesses/small-businesses-self-employed/manage-taxes-for-your-gig-work - IRS: Self-employment tax (Social Security and Medicare taxes).
https://www.irs.gov/businesses/small-businesses-self-employed/self-employment-tax-social-security-and-medicare-taxes - IRS: Topic no. 554, Self-employment tax.
https://www.irs.gov/taxtopics/tc554 - IRS: Form 1099-NEC and independent contractors.
https://www.irs.gov/faqs/small-business-self-employed-other-business/form-1099-nec-independent-contractors/form-1099-nec-independent-contractors - California Franchise Tax Board: 2025 Personal Income Tax Booklet.
https://www.ftb.ca.gov/forms/2025/2025-540-booklet.html
