Mtf or Ftm? I think most the damage is done for ftms by around 12 or so. Depends on each case. You may want to check with a doctor if your growth plates are done. If you have breast development, menstruation, and done most of your growing, not sure what blockers can do. If mtf, they may still help past a certain point. But check with a doctor. Make sure it's worth it for the price instead of just getting on HRT.