Junge does behave oddly since it's one of the weak masculine nouns, which add -(e)n throughout the plural and in all singular cases except the nominative.
In that case, den Jungen can either be interpreted as the boy (accusative masculine) or the boys (dative plural). However, since von triggers the dative case, we know it's the latter.
@Constructive : Correct your first sentence to this:
Weak masculine nouns add "-(e)n" throughout the plural and in all singular cases other than the nominative. See http://www.pauljoycegerman.co.uk/abinitio/chap11-9.html
Listening with headphones helps make a difference, although if you don't understand where you should use dem and den you might want to go over the dative case again.
This question isn't talking about one boy either, it's talking about a group of boys - you can tell by the placement of the apostrophe [boys']