There are a few reasons why. First,If one uses anabolics before reaching total growth potential and or sexual maturity(these conditions lean more torwards the sexual maturity aspect) you run the risk of stunting bone mass growth,also negatively impacting your growth hormone production. Most teens who use anabolics before their bodies are ready, suffer from severe joint pain due to early stunting of bone mass. For someone in your age group,I dont see anything wrong with it. By now your testes are not producing nearly as much testosterone as they were when you were younger,making it difficult to gain good mass.