Is the repentance of someone who stole when young and unaware of the prohibition accepted, then regretted it and felt too ashamed to face the person from whom they stole?

Repentance is valid by returning the money to its owner or seeking his absolution. If you know the amount, you are obligated to return it. If you do not know, then strive to determine the amount that you most likely believe is due and return it to the shop owner. It is not a condition to inform him that you deceived him.