The top high school draft picks are making impacts on MLB Clubs at the age of 20. There is a preference for players who can be contributors as young as possible. As the OP said, you better start dedicated lifting at 14-15. If you wait until you are 18 or 19 to start putting on 20-25 pounds of muscle you are way way behind the curve. Especially in 2019. And I bet, even moreso, in coming years.