I think they wanted to keep them 16 and Kevin 17 to keep the teen hero vibe and keep him from aging. Personally I wouldn't mind if these series aged Ben up to 17 or 18. It would have made Gwen leaving for college not feel a bit weird. Granted some people do graduate early and get into college early, but still feels a bit off.
I think the writers need to tone down Ben's arrogance a bit and mature him a bit. It is odd how the first season of AF is when Ben was his most mature. After that he kept regressing into a big headed and self-centered hero. Omniverse does show Ben's better qualities from time to time and had a episode that acknowledges Ben needs to grow up more and have Ben himself acknowledge his behavior isn't acceptable in the form of Ben 23 and mentoring him for a bit. I would say the writers are toning it down some.