Creatures with the young template are immature specimens of the base creature. You can also use this simple template to easily create a smaller variant of a monster. This template cannot be applied to creatures that increase in power through aging or feeding (such as dragons or barghests) or creatures that are Fine-sized.
Find the die to be decreased in the + column then look to the left (-) column to determine reduced die size for determining damage.
For example, a Medium rapier deals 1d6 damage normally. Find 1d6 on the table in the + column then look in the adjacent - column and see that it reduces to 1d4 when it is size Small.