You must file taxes if you earn the following amounts of income:
Self-employed, any age: $400
Children and Teens classified as a dependent: $5,700
Single, under 65: $9,350
Single, over 65: $10,750
Married, filing jointly, both spouses under 65: $18,700
Married, filing jointly, one spouse over 65: $19,850
Married, filing jointly, both spouses over 65: $20,900
Married, filing separately, any age: $3,650

Source: TurboTax Support website (related link below)

Even if you do not have to file, you should file to get money back if Federal Income Tax was withheld from your pay, which if you were an employee most certainly happened or you qualify for any of the following:

• Earned Income Tax Credit. The Earned Income Tax Credit is a federal income tax credit for eligible low-income workers. The credit reduces the amount of tax an individual owes, and may be returned in the form of a refund.

• Additional Child Tax Credit. This credit may be available to you if you have three or more qualifying children or if you have one or two qualifying children and earned income that exceeds $11,300. The Additional Child Tax Credit may give you a refund even if you do not owe any tax.

• Health Coverage Tax Credit. Limited to certain individuals who are receiving certain Trade Adjustment Assistance, Alternative Trade Adjustment Assistance, or pension benefit payments from the Pension Benefit Guaranty Corporation.

According to the IRS:

Whether you have to file a tax return depends, in part, on your filing status, age, and gross income.

A full 50% of the people who do not have to file actually get money back by doing so!
There really is nothing bad that can happen by filing, even if you don't need to, and in fact only good. (For example, the time limits for auditing and questioning starts as of filing, so if you don't file you can ALWAYS be questioned).

2007 1040EZ instructions state:

For most people if you are single you must file if your gross income was at least $8,750. If married filing jointly $17,500.

While these are the instructions for making estimated payments, they're good guidelines.

Simple Common Sense:
It really makes no difference since the only time you actually do WANT to file is when the IRS says you don't have to!

They don't do that because it's good for you. They do it because it is more likely to be good for them. Certainly if you don't have to file, NOTHING BAD, in fact only good things, can happen by doing so.

Federal Taxes are the same throughout the country. State tax laws are specific to each area.

Whether you have to file a tax return (or pay tax) depends, in part, on your filing status, deductions, amount & type ofincome. There are no such things as "start and stop" ages, not having to pay because of retirement or on social security or working from home or a student. It is all addressed as a matter of"how much TAXABLE income."

(Note: working isn't relevant either, as many people who don't work or are retired, or disabled, or old, or young, or in school, have income from many sources: savings, investments, etc. TAXABLE income is different than what you may otherwise think of as income. In most circumstances, you have to do many of the calculations needed to file a return, just to determine what taxable income may be).

Likewise, there are no special or fixed rates for retired, student, doctor, sanitation worker, President, convict...whatever. The amount of taxable income after applicable deductions and adjustments determines the rate applied to your particular situation. The rate, as well as the amount, you pay changes as the amount of income does.

You must file a tax return if you had net earnings from self-employment of $400 or more. This is your total self-employment income less the expenses paid in operating your trade or business, multiplied by 92.35%.

If you weren't self-employed (paid on a 1099 or ran your own business) then you would always want to file a return to claim the amount withheld and shown on your W-2, which with lower incomes will always be refunded to you.

If you are an individual who may be claimed as a dependent on another person's return, you are subject to specific filing requirements. Refer to the instructions in your tax package or refer to Publication 929, Tax Rules for Children and Dependents, or Publication 501, Exemptions, Standard Deduction, and Filing Information, for the filing requirements for dependents. All available at IRS.gov.

You must file a tax return if you received any amount of advance earned income credit payments from your employer during the year, or if you owe any taxes, such as:

• social security tax and Medicare tax on tips or group life insurance,
• alternative minimum tax,
• tax on qualified retirement plans including an Individual Retirement Account, or other tax-favored account,
• tax from recapture of an education credit, investment credit, low income housing credit, federal mortgage subsidy, qualified electric vehicle credit, or the native American employment credit.

Generally, you must file a tax return if you are a nonresident alien with income from sources in the United States. For more information on nonresident aliens, select Topic 851 at the IRS website.

Even if you are not required to file a tax return, file a return BECAUSE MANY, LOW INCOME PEOPLE HAVE MANY BENEFITS COMING THAT ARE KEYED TO FILING A RETURN. (Like stimulus checks).

Also, the Statute of Limitations for when the IRS can no longer ask you questions about your affairs for a year only STARTS to run when a return is filed. Not filing, and they can bug you, (and assess a tax) for forever.

You can file for free at the www.irs.gov site too.