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1. The State you were in when this happened and the State in which you are applying for a job
2. The specific questions on the background questionairre
3. The department for which you are applying.
Some states have laws in place to protect juveniles who commit crimes from being branded a criminal the rest of their lives, because juveniles usually grow out of the criminal life.
If the background question only specifies "arrests" or if it says something to the likes of have you ever been questioned by a LE officer.
Some departments just don't care or hold crimes committed by juveniles against them (of course there are exceptions to this, ie. murder, drug trafficking etc). You also may have had everything relating to that particular incident sealed by the court.
In all honesty 14 years is a long time ago. YOU WERE 12 for God's sake, only a moron would hold this against you.
I think it best that you put this down anyway, but if you don't need to or are not asked it then there is no need to volunteer it either.