oh my goodness it's all up to me....Whatever Sophie decides is happening with Kes & Neelix is what I will finally go along with ...
In regards to their ages...When Neelix first meets Kes she is at the most just over a year old since we see her 2nd birthday in the early 2nd season. We don't know much about when Ocampans reach maturity but physically she clearly appeared to be an adult. So as a human comparison that would be probably between 16 and 18. Physically an adult but mentally not really. The Caretaker treated them as children, sheltered them and they didn't know anything about what was beyond their underground cavern. In many ways Kes and all of the Ocampans were naive children.
We never get a clear indication of Neelix's age, but I kind of always put him 40ish based what he's said about his past and all of the different jobs he's held.
I honestly don't think that Neelix ever intentionally manipulated Kes into a relationship, but I do think there was some kind of "manipulation" going on. Neelix saw a young beautiful girl being abused and wanted to help her. Many people would feel sympathy for that. Maybe she reminded him of his dead sister or lover he had before. Kes, coming from underground not knowing anything saw someone from another world, who saw and knew things, who was kind to her, who saved her, and she fell for him. That makes sense too I think. She literally didn't know anything else. So when she did come to Voyager and learn more and become exposed to more cultures it makes sense to me that she would break up with him to try to experience other things.
I think I've said this before, I would have rather seen their relationship be more of a father-daughter one than a romantic one. I think that would have worked better than a 40 year old guy and a 17 year old girl. And as I've said before once they broke up and Kes left I began to like Neelix a lot more as a character, he became much more interesting and had more depth in my opinion.