"The angry old man hits the lazy young man."
Translation:Senex iratus iuvenem ignavum pulsat.
-um is the masculine accusative singular ending so it agrees with iuvenem. -em is a third declension ending so iuvenem uses it but ignavum follows the second declension.
Adjectives matchs a noun in gender, number, and case but not declension.
ignavus is a first/second declension adjective meaning the feminine forms follow the the first declension, while the masculine and neuter forms follow the second declension.