It is possible to make a teen elf look fierce and brave. Not only elders seem serious and powerful, it is mainly about shape language and expression. To seem brave, to seem fierce, we associate facial and emotional expressions with that.
It can also be a contrast to the current situation. Their expression against the dangers they face, showing their courage. That is also a way to interpret such in characters, regardless of age.
Simply put, if you want to make a fierce looking young elf, you need to make sacrifices. The young elf can't be all that petite, because that is almost the opposite of fierce. The elf needs to be sharp, have an almost sinister look to his or her shapes, with very robust forms and edgy, sharp endings. This can also be used when creating expression, to give off even more of that fierce, brave look. Besides that, you might have to give the character scars or items that reflect exactly that. Maybe the character carries knives to defend him or herself, or a training sword made of wood, showing that he/she wants to protect, without hurting others.
In the end, it is all about character, and story. Try thinking of it logically, and you can cheat your way into making the character look brave and fierce, yet young.
Anyway, I hope this helps. And I recommend to make your choice very obvious. If you try merging petite and fierceness, you might get a result that is neither, and since both are almost opposite, they cancel each other out, making a confused character, that we can't read.
