A level 12 warrior wearing a level 51 +5 damage roll breastplate and a level 51 warrior wearing the same breatplate both see the same affect (i.e. +5 damage). You could probably argue that the lower level warrior actually gets more benefit (5 hp of damage being a bigger % of the mobs and pc's he is fighting overall health)

A level 12 vs. a level 51 warrior will definitely see a difference using a level 51 weapon. But it isn't as straightforward as damage * 12/51 so if that warrior lucks upon a powerful enough weapon, he is still going to hit pretty hard.