Just what the title has to do with the movie is of some mystery, one that might be unlocked by those who pay close attention to the movie (and if you are one of those people, you really should see more movies). Perhaps it has something to do with the fact that Rafi Gardet (Uma Thurman), whose name sounds like something you might order with a side of hummus, is an "older woman" in her sexual prime -- older being her late 30s, and only studio executives would classify this as being older. (No wonder Shirley MacLaine was relegated to the final half of the Cameron Diaz-Toni Collette sisterhood pic In Her Shoes; in movie-studio years, MacLaine's been dead since 1997.)
This would explain why Rafi, just divorced from a man apparently her own age and possessing the libido of a corpse, winds up with a much younger man named David Bloomberg (Bryan Greenberg) -- much younger being his early 20s, though Greenberg and Thurman look all of nine months apart. Rafi needs a young man to satisfy her needs. Men her own age apparently don't have the stamina to fuck her all night on every surface in her apartment, which is an issue writer/director Ben Younger, already a creaky 31, might need to take up with his therapist or a trainer or a stack of self-help books.
Or perhaps the Prime in the title is intended to signify the twosome Rafi and David become against her better judgment and his mother's wishes, his mother being played by Meryl Streep as though she's auditioning for a summer-stock production of Fiddler on the Roof. See, two is a prime number. So, there's that. David and Rafi have more than their age difference acting as a barrier between newfound lust and long-lasting love; their religions, too, are working against them. He's Jewish, she's not, which ain't kosher with David's mom, Lisa, who's seen either noshing on giant corned beef sandwiches on her apartment rooftop or sitting at the Sabbath dinner table on Friday night, alongside her husband and parents, with whom David also lives. This movie's so Jewish that come the year 2018, it'll have to get bar mitzvahed.
But there is one complication even bigger than the age and religious differences: Lisa is also Rafi's therapist, a fact Younger blessedly doesn't keep concealed from the audience for too long. Lisa figures this out long before Rafi or David, however, which makes her not only a lousy, conniving mom, but also perhaps the most unethical therapist in New York City. Lisa's guilt-ridden over the deception, of course, but not enough to stop Rafi from describing the beauty of David's penis during their myriad sessions that devolve into sex chat. Younger, maker of the overheated Boiler Room some years back (notable as one of the few movies ever to put Vin Diesel in a tie), throws more roadblocks in front of his lovers than a state trooper.
But the problem with the discrepancies in their age is a cheat; theirs is a June-July romance, at best. Younger might have earned his tension by casting an older actress, but Thurman, at 35, has the mien and temperament (and wardrobe) of someone far younger; and Greenberg, playing a painter of intimate, widescreen portraitures, carries himself as someone far older. And Lisa's less a concerned mother than a pain in the ass; the woman loses all sympathy, and credibility, the moment she betrays her son and her patient, yet still Prime demands we think her caring and loving. Younger, for whatever reason, simply can't abide the couple's happiness, and so he destructs the relationship from time to time for no reason, using plot devices that wouldn't have been out of place in episodes of Three's Company. (One involves David's slacker-schmuck pal Morris, played by Boiler Room's Jon Abrahams, hiding in a closet, which angers Rafi ... why?) His is just more conventional schmaltz, served up on a paper plate.