No, not unless you work for an agency of government, or in a position which requires a security clearance.
probably, but when you turn 18-21 all of your juvenile records are destroyed
Yes, it can. Juvenile records are only sealed from the general public - NICS and law enforcement agencies can still access them.
That depends on what charges are on your juvenile record.
If the job requires any kind of security clearance or background check for any other reason, they can find out the reason why it's denied if it is because of something on your juvenile record. Otherwise, no.
(in the US) your juvenile record becomes sealed after you turn adult. However, your adult criminal record is a permanent history and can only be changed by expungement
Forever, if you don't get "charged" again :-)
No! You absolutly do not have to divulge your juvenile record.
If you were charged as a juvenile, your juvenile record will automatically become sealed to the general public whn you reach your 18th birthday. HOWEVER - the courts, law enforcement, and government agencies will ALWAYS have access to your true record even if you get it expunged.
You do not need to generally report an expunged record on your job application. It is erased as far as the court is concerned and no one should have access to it.
The courts and DO access your juvenile record and a judge CAN access them in order to gain background knowledge about you or form an opinion as to whether or not to offer you probation or to learn what kind of past record you have.
As a general rule, yes. After your 18th birthday your juvenile offense record is sealed to THE PUBLIC. However, it doesn't disappear - law enforcement and the courts still have access to your record.
Once the juvenile reaches the age of adulthood, their juvenile record should become automatically sealed to the public. I am not aware of any specifc procedures for expunging a juvenile's record while the individual is STILL a juvenile.