It has nothing to do with age. It's just a situational thing. First of all, Kirk, Spock and McCoy have no reason to think that the daughter has any possible way of knowing of her father's past sins. So, there is no motive for her to do it. Had they suspected she knew, then suddenly a strong motive would be there, and then she might be a suspect. That's the biggest reason. But, then there is personal bias. Kirk is at first using her as a tool to get to the father. He has initially biased himself from the start that she is not a suspect. Actually, the thought never enters his mind. Then later he finds her attractive which adds to the bias. Add to that her stunning beauty and who will naturally think of her as a murderer. Maybe a viewer of the episode would suspect her because it is a "who done it" and it is natural to start suspecting everyone, especially the unlikely people. But, you have to place yourself in the shoes of the characters to understand how Kirk got blindsided.