While it is true that certain dietary deficits take years to surface in actual somatic issues and probably as well that the majority of vegans is younger than the western average, neither is evidence that a vegan diet (especially a modern one) can’t be sustainable & healthy for a whole lifetime.

Large fractions of the world’s population don’t eat diary products at all (like said before, lactose tolerance is rather the exception than the rule, globally), and others don’t have access to meat/fish, at least regularly, can’t keep certain animals or what not - do you consider all that unhealthy?