At the very bottom of the pyramid, there is a league outside of the contiguous United States in the Dominic Republic, the Dominican Summer League. Populating that league are the youngest professional players in the Miami Marlins major league affiliate system. The Cuban National Series in Cuba generally has a few 16-year-olds as well.
These players, including some not yet high school graduates (that’s why it’s a SUMMER league, duh), are amongst the least refined talents in pro baseball. There are currently 15 players in the Miami Marlins system under the age of 18-years-old.
The DSL starts generally around the beginning of June every year, this year on exactly June 1st.