The Laws of the Game - Age Grade Rugby
The Laws of the Game (not Rules!) are determined by what was known until 2014 as the International Rugby Board (IRB). It is now known as World Rugby
The Laws include Variations and some of those apply to U19 rugby
The RFU has further local laws that apply further down the age grades - so it can all get a bit confusing
The best advice is to purchase a copy of the Laws of the Game.
However you can go to the World Rugby site to read The Laws of the Game