Let's take into account that Spira didn't have any modern medical technology like we do in the real world.And Sin went around killing everyone and destroying everything. The life expectancy for Spirans may have been very low and 17 might have been the equivalent of an adult. Besides, aged 17, raging hormones, same with Tidus. It was bound to happen. And it happened in the cutest way possible. I thought she was a pleasant character, I don't think I would have been too happy marching off (SPOILER)to die but Yuna did it with a smile and in my opinion, that's very admirable.
I thought her boots were cool.
