"The girl eats with the boy."

Translation: Das Mädchen isst mit dem Jungen.

[Max2013]

Why is "Jungen" used here? Why not "Junge"?

[deactivated user]

"mit" takes the dative case.

[martinblixt]

Which is why the article is 'dem'. Do masculine nouns always end with 'n' in the dative case?

[grubymis]

No, there are three declension types in German: weak, strong and mixed.
Some nouns (for example: der Junge, der Affe) declines with "weak" pattern:
Singular
N. der Junge
A. den Jungen
D. dem Jungen
G. des Jungen

You can find everything about German declension here:
http://german4u2know.tripod.com/nouns/04.html