According to the federal government, there are seven very specific criteria for being considered independent for financial aid purposes. It doesn't matter if you've been on your own and paying your own way since you were 15. Here's what the government has to say:

If you meet ONE of the following criteria: you are considered independent if you
are 24 years of age or older by December 31 of the year you're applying for aid
are a full orphan, ward of the court, or was a ward of the court until the age of 18
are an armed forces veteran
are a graduate or professional student, regardless of age
are married
are a parent yourself, or have legal dependents
receive special consideration from a financial aid officer who will document your status (VERY rare to get)