when you sign up for google adsense they do not ask you to fill your birthday... so how they know if you are at least 18 years old ?
Everyone can do online markeing, so google accept everyone. If they can follow google policy, google does not care.
even if they ask, it is possible to enter anything. its just that no company can make an agreement or sign a contract with a minor so if you say you are of legal age, thats all matter for some company.
The only way google can tell is if you send in a photocopy of an ID that has your name, birthday and your address. Seems like too much work.
If you're from USA, you need to fill in your SSN to get paid. This is reported as taxable income to IRS. Hence, 18yrs. old is minimum. If you're under the age and you enter your SSN, they'll get you.
Why? Google has no way to determine your age from your SSN. It is not illegal to 1099 someone under the age of 18. There are state laws setting limits on the type of work and the number of hours per week that minors of various ages may work, but that's not relevant to this type of income.