The halacha says that until a person is 20 years old, he is not mature enough to sell real estate that he inherited from his father. How much more so is such a person not mature enough to give away vital organs, parts of himself that Hashem gave him in case he ch”v needs them. So definitely wait until you are 20, and only then start considering it seriously.
Organ donation is a mitzvah but it’s not for everyone. It’s a major step that will limit your options for the rest of your life, so once you are 20 you must be very very sure this is what you want to do before you do it. Consult wiser people, people with more experience than you, talmidei chachomim and tzadikim, and pray for Hashem to guide you to the right decision. And if the result of all that deliberation is that you truly believe this is what Hashem wants you to do, then behatzlocho.