He knew about the functionalities of puberty long before he was thirteen but Rosinante deemed it rather necessary - a personal goal of his, perhaps - to attempt to inform him of the changes his body would be partaking the moment Law managed an undignified squeak to his words - a fact that would be the least bit embarrassing if not for the fact that Roci made such a bigger deal out of it than absolutely necessary.
After having been cooed at, Law decided that he would give up speaking altogether until his larynx sorted itself out - however long it may take. But, it turns out, his traveling companion didn't stop in his would-be intellectual crusade. Rosinante took it upon himself to measure out Law's height in the most round about way possible: standing next to him and judging if he grew magically overnight based on where the top of his head ended up in position to his leg. He even started drawing crude lines on the length of his pants, much like you would a doorframe, to Law's utter dismay.
"Look! Law, you grew!"
"You're just wearing your pants differently today. This isn't a reliable system, Roci, stop this."
A gentle gasp. "Your voice!"