Ok so far? Now, what's green fleet? Green fleet is a special fleet for beginners. Green fleeters can be any age up to 15. A green fleet might have skippers who are as young as 6 and as old as 15, but sooner or later they'll move into their appropriate fleet. A ten year old green fleeter would move into white fleet, a thirteen year old green fleeter would move into red fleet, etc.
The purpose of the Green Fleet is to encourage novice sailors. The Spirit of Green Fleet is to ensure that novice sailors do not get overwhelmed by the more experienced, aggressive sailors and are able to sail at a level that they feel comfortable. Once a Green Fleet sailor obtains a level of proficiency they would be expected to move on to the Championship Fleet races (White, Blue or Red depending on their age). Read here for more info on the Green Fleet.