Michael Sicinski’s review published on Letterboxd :
I recognize that I am the target demographic for this movie, in the sense that it always takes a minute for the title to register as appropriately strange. Oh yes, we are no longer in the 20th century, and haven't been for 16+ years now. And although I will (inch'allah) live the lion's share of my own life in the 21st, I still feel acutely like a 20th century subject.
In particular, this film is a mostly positive spin of the 1970s and the post-Summer of Love ambitions many people had to care more openly for one another, to practice a saner form of politics, and to find ways to reverse the wrongs of history. This is, in essence, the anti-Ice Storm, a small corner where Dorothea (Annette Bening) could stumble as a mother but still be fundamentally good, not have a "village" surveilling her every move, not be shamed into conformity (or conformist nonconformity) by the blogosphere, and could be hopelessly idealistic enough to find President Carter's "crisis of confidence" speech "beautiful."
(If we want to talk about "Obama Cinema," this is it -- Carter as the uncool, as yet unformed kernel of an idea that Obama would bring into the mainstream, that you could have a politics of non-machismo in America instead of the no-apologies, fuck-yeah swagger of Reagan, Clinton, and Bush 43.)
This film is said to be partly autobiographical. This explains, I think, why so many aspects that could feel precious or glibly symbolic in other contexts come across here as lived-in and effortlessly wise. This may be unfair, but if you compare Mike Mills' cinema -- Paperboys, Beginners, this...everything but Thumbsucker, really -- to the films of his partner Miranda July, you'll see commonalities of approach. But July always seems to be to have one foot on the floor, as it were, never letting go of art-world irony enough to embrace ordinary feelings. That may be July's preferred mode. Like Laurie Anderson, July operates at a remove, cultivating a certain distance. But Mills shows what happens if you apply similar concepts within a more direct emotional register.
This disarming aspect to 20CO+, I think, helps obviate the smug criticisms that viewers so often like to lob at historical time-capsule works. Yes, we "know better," although we have no better grasp of the times in which we live and breathe. But we completely understand Dorothea from the register of her times. She is concerned about raising a son. She doesn't know if she can, or if she wants to. She knows that she loves Jamie (Lucas Jade Zumann), and wants to raise him to be a good man. But her 70s feminism leaves her unsure what a good man looks like. This is part of her absolute love for him, wanting to protect him from patriarchy, a force she sees as waiting to deform him, rob him of his sensitivity. (There is no "war on boys" nonsense here.)
Likewise, she takes a risk of allowing other women to have an impact on Jamie, even if she cannot be certain what that impact will be. Even for me, it was hard watching the relationship between Jamie and Julie (Elle Fanning), because it seemed like the terms were not equal or clear. In most other eras, a parent would simply deem such a girl "toxic," and demand that she stay out of their son's life. Or, we can imagine even worse outcomes, based on Jamie's frustration. But there is a tacit assumption that both Jamie and Julie are decent enough kids that no one will get seriously hurt, and both parties may learn something in the bargain. (What if all boys today had to really get to know a girl they pined away for, and ultimately respect her decision not to be with them? This could change so much.)
Greta Gerwig's Abbie is possibly the most unambiguously positive force in Jamie's life, serving as a big sister. And William (Billy Crudup), a benevolent lunkhead, provides something resembling a male role model -- dudelike but harmless. But much of this becomes beside the point. Mills uses Dorothea's desire for a group-parenting of Jamie as a starting point, but in a way it is just a pretext for building an unconventional family.
All of these women, Dorothea very much included, have fully formed lives apart from their relationship to Jamie. Abbie's cancer and inability to have children, for instance, manifests in her dueling tendencies toward a young artist's identity and a desire to get her life sqaured away. Her sexual encounter with William shows how pliable he is, and how much she wants to be seen, discovered as someone new. If and when we listen to Julie -- something Jamie cannot always do -- we hear her ambivalence about being the "cool girl," not necessarily wanting the maturity she feels she must perform.
And above all, we watch as Dorothea's brash feminism ages into cautious motherhood and an unexpected suburban conservatism. She becomes prudish about sex, starts to think that Julie, William, and especially Abbie are exposing Jamie to too much too soon. But the turning point of the film, really, is an interaction between Dorothea and Jamie. After reading various books on feminism, Jamie reads his mother a passage that struck him as poignant, and made him feel a tinge of deep empathy with her. It was about how society discards women of a certain age. Dorothea, unprepared to hear this from her young son, shames him. "You think you understand me now, because you read a book?"
From this point forward, she doesn't exactly support Jamie's growth as a pro-feminist, sexually aware man. She doesn't always stymie it, but her enthusiasm dampens. Jamie clearly didn't intend to mansplain, but Dorothea's reaction is potent because in a way it is Mills setting the stage for the Reagan 80s. Like so many others, it seems, Dorothea thought she wanted new forms of thought, but when they hit too close to home, chose instead to retreat. Add together enough of these small private slights, confusions, and resentments, and you have a tragedy.