It usually comes with a certain level of maturity I believe, because even when someone has all the evidence laid out in front of them, they can always find some shadow of a doubt answer and carry on the same way, even if it is cognitive dissidence.
I’m sorry to say, it may just have to wait until they are a bit older for them to appreciate the discussion and take it seriously. At least that has been my observation of many people I know who went through the same/similar situations.