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Juvenile records may have "restricted access" after age 21, if there have been no other crimes between the age of 17 and 21. As a sex offense, it can be restricted - which means it is not available to the public but only to certain law and court officials.
It also is available to certain employers, such as if you are applying to work with children or similar agencies.
Also, if your crime required registration then you will need to Petition the Court for the expungement of the record, but you can only do that once your registration period has expired.
If he wants to have the matter expunged, he can Petition the Juvenile Court, yes.
Or you can wait until he is 21 and it is automatically restricted. But if you want to have the matter considered for expungement, then file the Petition with the Court who adjudicated the matter