1. Baby-sitting: If you earn under a certain amount of income in a year, you are not required to file a return at all. Most teenage baby-sitters will fall into this category. A "professional" baby-sitter would have to obey local rules on having a business license for self-employment and file a tax return as a sole-proprietor, much like what most of us do.
2. Regular work paid at $15/hr... This will often come from a small business who should be reporting you as an employee so that they can deduct your wages. If they are reporting you as such, then you had better report the income as the IRS will likely catch the non-report. If they are not reporting you, they are in danger of getting into trouble should their finances ever by audited. If the entity is not registered as a small business, they likely should be.
So... Legally, yes, your income should be reported. However, I don't equate "legal" with "moral." So, is it immoral to not report the income? I know that I get frustrated when extremely wealthy folks find all sorts of "loopholes" so that they avoid paying taxes. I strongly believe that, as they are benefiting from our society, they should be paying taxes on their income so as to help support the system that enriches them. By extension, I believe that we are all morally obligated to pay our legal share of taxes. Do I agree with all aspects of the tax code? No. However, we live in a democratic society and we have to live by the decisions made with our democratic processes. If I could refuse to pay taxes because I disagree with the tax code, how would I argue that somebody else should pay? What right would I have to demand that others pay taxes if I were to create what I view as the "perfect" system of tax laws, should they disagree?
Report your income within the parameters of the law.
Hard work builds character and homework is good for your soul.