Two things. 1: Why have a specific age counter in addition to the body type section? The "X year old" would've fit right along with the teen/adult descriptors. 2: I noticed that both jackbunnies and goblins have a difficulty range of 1-6 despite goblins having the more valuable materials. Do you have plans on increasing encounter difficulty with future monsters, and if so how? The most likely I can think of would be 1-6 --> 2-7 --> 3-8 ect., but I could also see 1-7 --> 1-8 and 2-6 --> 3-6, as well as a mix.
The age counter thing is just to add flavor to some scenes, a personal preference really. It serves no real purpose.
I hadn't really thought of increasing the difficulty of encounters, but your suggestion is really good. I might implement something along those lines if I think it could work. It is not a bad Idea since the PC has gotten stronger as the game has developed while the monsters have stagnated.