Robert Heinlein spiked this one into the end zone as he dropped. What an ending to his career.
(The title is a reference to a line in Tennyson's 'Ulysses', having to do with accomplishments in old age, and it's undoubtedly intended to describe what Heinlein himself was up to here. He succeeded.)
A word of warning, though -- if you didn't like _Time Enough for Love_, stay away from this one. Even (if possible) more than its predecessor, this one just oozes s-e-x, including wife-swapping, incest, and other stuff probably not in conformity to the mores of your tribe. In my view, it's all very tastefully and responsibly handled, but then my own opinions on such matters (including my devout antigrundyism) were in large measure informed by massive reading of RAH during my formative years. Just be aware that the usual suspects have dismissed this novel as pornographic trash.
At any rate, this novel was clearly a labor of love for Heinlein. In it, he gets to revisit the world of his childhood (or close to it; he actually has to start a bit earlier than his own birth).
You see, it's the story of one of Heinlein's most compelling heroines: Maureen Johnson Long, of the Howard Families, mother (and co-wife) of Woodrow Wilson Smith (a.k.a. Bill Smith a.k.a. Ernest Gibbons a.k.a. Lafe Hubert a.k.a. Aaron Sheffield a.k.a. Lazarus Long). And she lived just down the road a piece from Heinlein (and Sam Clemens, who makes a nice cameo appearance in her memoirs).
There's a thin shell of story around it, but most of the novel consists of Maureen narrating her life to herself (and us). We learn a lot about her unconventional childhood and her interesting relationship with her father (Lazarus's Gramp, Ira Johnson).