Short answer, it depends.
No matter what happens, the mere fact that you were arrested will always be true. 50 years from now when a potential employer asks you if you've "ever been arrested" - that answer is going to be yes. No matter what happens after the arrest, be it conviction, acquittal, or dismissal. The arrest still happened. Period.
If you are a juvenile when you are arrested, and you get convicted, it will be on your juvenile record. Most criminal agencies will not use this to enhance any additional punishment for a crime later committed as an adult.
For instance, if you had a DUI as a juvenile & then get one as an adult, you shouldn't be sentenced to a second offense because your first occurred when you were a juvenile. This would not be true if you were an adult when you committed the first DUI, in this example. If you have multiple alcohol related priors as a juvenile, though, and then get a DUI as an adult - while the priors as a juvenile might not be used as a mandatory sentence enhancement, common sense tells you there is a problem with alcohol. Your sentencing on the DUI as an adult may therefore include a little more focus on alcohol, even though the juvenile priors (generally) can't be used in a trial against you.
If you are an adult when you are arrested & convicted of a misdemeanor, it remains. You may file a motion to set aside the judgment, which is a way to expunge the criminal misdemeanor conviction from your record. Some charges may not be "set aside" no matter what, and the view of most prosecutorial agencies is this is only an option if you have fulfilled the sentence & a year has gone by without any problems. For example, you get convicted of shoplifting and are sentenced to a year of probation & a $455 fine. In this example, most agencies would oppose this "expungement" until you pay the fine, the year probation ends, and a year has gone by after probation ends.
When this is granted & someone does a quick background check on you, they will not find it. When they do a detailed criminal background search, they will see the charge & conviction but that it was set aside.