Legally, no. Morally, it would depend on their personality. Your prefontal cortex, which is responsible for so much of what makes a person an "adult" doesn't even finish developing until the age of 25. Society can put all the specifics on it they want, adulthood is a very individual thing.
depends for what but generally no
Depends on how they behave. Age is a number; behaviour isn't.
If you kill someone you will be
No. Neither would I consider that proper grammar.
Still not an adult.