Once you have committed a crime, it shows up on a criminal record for the rest of your life. The employer doesn't 'go back' they simply look at the record.
Not true. It depends on the state. Some state laws go back into your criminal background check 10 years, others only five. There are different laws according to the state that says how far back you have to disclose your criminal record. I.E. If back in '81 you committed a grand theft auto, most states don't require you to disclose it. You should look it up in the state statutes. Good luck.