Lucy, when KinezumI started her "age" thread, I browsed through the FishLore rules to see if there was anything about a minimum age. I didn't see anything. I've heard the age 13 minimum on a lot of sites, but I didn't know it was actually a law; thanks for the links.
Just a suggestion, maybe the FishLore rules could explicitly mention the minimum? Young children may not be aware of the minimum age when they join. Assuming the rules don't already say it, of course -- I might have missed it.