need translation confirmation

i had a buddy translate this for me, but i wanted to be sure that she did it right. in english it should say approximately: you're going to be the world's toughest 15 year old.
and this is how she translated it: 君はこれから世界で一番タフな15歳の少年になる

does that say what i want it to?

Yes, it does :) provided that you are a boy (it says "15-year-old boy" in Japanese), and your intention is to say that "in the future" you "will become/will grow into the toughest 15-year-old." What I am trying to say is that the English expression "are going to (be...)" can be used loosely to describe either future or present situations. In Japanese, here it says "You will, going forward, become the toughest 15-year-old boy." So if that's your intention, yes, it is accurate.