First of all, I am not dismissing the possibility or accuracy of this prophecy, but I do have to wonder who told you of it. I highly doubt that you will die when you turn 18 because of reserved magical energy residing within you. I am an Oracle, and I have prophesized a many, but never any quite so.. well ridiculous in my opinion. and I apologize if I am insulting anybody by saying so. But if it will make you feel any better I will divine the mysteries for you and see what I can.
However I honestly think that whoever said this is pulling your chain. Or perhaps they are jealous of you for some reason, and may be binding you in some way, in which case that can be easily be resolved.
If after you read this and you do not agree, and wish to seek further help, then by all means contact me.
Blessings and peace be with you,