This Slate discussion seems sort of backwards:
I liked the earlier years of Boyhood when it felt like an ensemble picture, though the parents, too, got less interesting thanks to the film’s message that eventually we all become square and middle-class. But as the focus narrowed on Mason, I completely lost interest. That ending with that sunset felt like unearned, banal triumph—congrats, kiddo, looks like you’ve got a reason to put a sock on your dorm-room doorknob.
This is the danger of bringing a little too much to one's viewing. The movie read to me in the opposite way: the parents become more interesting as they get older, since the son gets a progressively larger view of them. Ethan Hawke isn't cool anymore, but he's a much better father, more grounded and mature, who has opened himself up to experiences whose validity he had previously denied (he's clearly somewhere on the religious map, if not as crazy as some of the relatives). The mother is an accomplished professional who gets to do work she loves, and the focus is at least as much on her as anyone--she gets the pleasure of seeing her son succeed, all that work coming to fruition, and a real moment of actual pain at losing him to college. What the parents are no longer is godlike, inexplicable beings: they're people. And people are both deeply interesting and a little boring.
The ending is, indeed, an "unearned, banal triumph," which seems as accurate a description of going away to college as one could imagine--it feels, significantly, like both the beginning and the end of something big, when it is (in reality) usually neither.