Let's say I lived in NJ for like 3 years when I was younger. Then I moved to a different state for 8 years. Then decided to apply to a school in NJ, would I be considered an in-state resident or na?
no, the whole point of in state vs oos is your parents pay tax for that state (which contribute to funding of that dental school) which in turn give you cheap/lower in state tuition
moving out of that state for more than 1 year you may as well give up your residency for that state already.
The state you live in is the state you currently have your permanent address in and have held for a specified amount of time - generally a minimum of one year.
Another example is that your permanent address is in Florida but you go to college in Georgia. Although you have been "living" in Georgia, you haven't been as a permanent resident but instead as a student. Therefore your state residency is still Florida.
