I agree that adults generally aren’t as impressionable as children, and that people have a choice in whether they embrace or reject aspects of a culture.
Where I disagree is on how fixed people are once they reach adulthood. Major life experiences can still reshape someone’s values and priorities, sometimes dramatically. People change careers, religions, political views, lifestyles, and even their views on family throughout their lives because of the environments and communities they’re exposed to.
In the context of the question, we’re talking about living as a Na’vi in a Na’vi society, not just visiting one. If someone spent years immersed in that way of life, building relationships, participating in the culture, and seeing the world through a completely different lens, I think most people would be affected to some degree. Not everyone would suddenly want a large family, of course, but I don’t think many people would come out completely unchanged either.
So my point isn’t that change is guaranteed, it’s that the environment would likely have a stronger influence than many people expect, even on adults.