For the IRS your dependent deduction drops at 17. You can't buy tobacco or alcohol till 21, 18 you are an adult on most things, but health insurance (as well as parents counting against college aid) is what 26. Most states you can get married well below 18, with permission (and or other special circumstances), and at that point, from a financial aid standing, they are independent.
It's kinda screwy, pick a dang number, if it's 18, it should 18 for all aspects, if it's 19/20/21, make that the number...
My oldest is going to be 19 here in March, and the bank still says he can't have a full fledged adult account till 24? He's grown, and there should be no need to have an affiliate adult on his bank account