Zero understanding? What's there to understand? The article isn't set to examine the quality of human relationships, or their intricasies, just their quantity over time.
It's also obvious, at least for spousal relationships, that those wont slow down over time, you'll continue to see your spouse everyday -- so it's clearly a relation outside of the scope of the article, which is relations were "time together" diminishes with age.
As for the children, while you'll continue seeing them everyday for a couple of decades after having them, after they're around 18 you'll have seen the most of them you'll ever will, even if you're only 40 and have 40+ years ahead of you. And that's for the same reasons mentioned in the article. Especially if they move to another town/state, but even if not.