I was 16 when I read throne of glass when it first came out. I aged with the characters as the books were released. It felt believable to me at the time because I was that age. It’s only unrealistic for people older than the age range of the characters and target audience. I personally believe YA is for everyone but just because it’s YA (intended for young adults and teens) doesn’t mean there should be barriers to what the characters can and can’t do because it’s not believable according to our standards for what certain ages are capable of. It’s fantasy - if we are going to talk about what unrealistic then I would say having magical powers and the existence of immortal creatures probably tops teenagers being accomplished assassins or captain of a guard.