but the question becomes: what is the best way to favor immutability [..]

To which my answer would be: it doesn't matter, as long as you consider it and try to apply/enforce it wherever it makes sense.

  I have found that age matters very little with regard to
  someone's ability to write code. [..]

I was thinking of their greater (life) experience; in my experience they tend to be more nuanced and open to different ways of doing things, which makes for nicer, constructive, conversation.