The amount one pays as income tax depends on their TAXABLE income. It is a percentage of that income. The exact percentage used depends on the level of that income.
Taxable income depnds on many things: Earnings from employment for sure, earnings from other sources (investments, government payments, etc.), and even then certain items of each may be not included, or things you may not receive in cash may be included. For example - the contribution to a 401k is not taxable income, even though it is part of your salary.
On the other hand, certain benefits you may receive, like employer paid life insurance, car allowances, even access to a cafeteria that has reduced prices because of an employer supporting it), may result in taxable income to you.
Once the amount of taxable income is determined, then the deductions to that income are applied. For example, interest paid on the mortgage for your home, number of dependent children, number of other qualifying dependents, medical costs, certain expenses of making that income, state taxes paid, etc.). Hence, any 2 people, holding the exact same job at the exact same salary and benefits, may well have 2 entirely different tax amounts due.
Once the amount of taxable income is determined, looking at the tax rate charts (made by filing status, for example single filer verses married filing jointly), for that income determines how much one actually must pay. THE AMOUNT ONE RECIEVES "BACK" AS A REFUND IS SIMPLY HOW MUCH THAT IS LOWER THAN THE AMOUNT THEY PAID IN AS ESTIMATED PAYMENTS - OR IN MOST CIRCUMSTANCES - THROUGH PAYROLL WITHHOLDING. You actually controll how much that was when you completed your
W-4, and hopefully it is about right fior the amount needed to be paid, or you incur penalties and interest.
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.