he fountain of youth has been an unusually fertile myth in the history of our age-averse species, serving as inspiration for everything from the early exploration of Florida to the greater part of the unsolicited e-mail that assails my in-box every day. But few people have taken the myth to quite the conceptual extreme that Andrew Sean Greer has in his second novel, ''The Confessions of Max Tivoli.'' For one thing, Greer's protagonist needs no arcane elixir to erase the ravages of time; the process of youthing happens to him naturally. It also, however, happens to him involuntarily, giving the hopeful old fantasy a dark new twist: born with the appearance of a 70-year-old man, Max must pass his entire existence regressing, with steady, year-by-year regularity, to a state of physiological babyhood. ''There is no name for what I am,'' he explains at the start of this alleged ''found'' manuscript about his life. ''Inside this wretched body, I grow old. But outside -- in every part of me but my mind and soul -- I grow young.''

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Those who find themselves already bridling at such an anatomically implausible premise should probably just move on to the next review. If you're the type of reader who needs aerodynamic explanations for Peter Pan's flying ability or (pace Nabokov) entomological justification for Gregor Samsa's metamorphosis, you will not be happy here. But if you have no trouble accepting the idea of a 10-year-old boy who seems to be a 60-year-old man (and who, half a century later, can pass for a 10-year-old boy), read on. ''The Confessions of Max Tivoli'' may have its flaws, but ordinariness is not one of them.

Like many a hero in fantastic literature, Max begins life with a symbolic bang -- and, in this case, a literal one. He is conceived in 1871 inside a deserted heliograph station in San Francisco at the very moment that Blossom Rock is dynamited in the harbor, freeing the Golden Gate of an obstacle that has imperiled ships for a century. Max's parents, a Danish emigre and a transplanted Southern belle, don't know quite what it is she gives birth to nine months later, but his father has a theory that it may be a Nisse, a mythical gnomelike creature that lives beneath the Danish countryside and brings good luck.

About this -- particularly the good-luck part -- the elder Tivoli is wrong. For when the repercussions of Max's reverse aging are eventually understood, the tragedy of his predicament becomes clear. Not only does he have the exact year of his death forever staring him in the face (1941, when he will complete his 70-year process of anti-decay), but he must also live his entire life, except for a few brief months in 1906 when his real and apparent ages coincide, being something other than what he seems.

''Be what they think you are,'' his mother advises, and Max tries to do just that, playing whatever role his physical appearance imposes on him. Of course, this kind of perpetual masquerade proves to be anything but a recipe for happiness. ''The Rule,'' as he calls his mother's dictum, forces Max to adopt an unnatural posture of deceit toward the world. While he does collect a few friends who know his true nature (including a childhood playmate and a former servant who becomes one of San Francisco's most celebrated brothelkeepers), to the rest of his circle of acquaintance he must remain a stranger.

Max's affliction does, however, have at least one advantage. After finding the love of his life at an early age -- Alice, the charismatic 14-year-old girl who lives in the apartment downstairs -- he is able to bounce back when she rejects his moony teenage fumblings as the lecherous advances of a dirty old man. In fact, he is able to woo her twice more in his life, at roughly 20-year intervals, since each time their paths cross she cannot recognize this ever-younger person as the man she knew before.

Max, in other words, gets three shots at winning his one-and-only love, and while none of these courtships work out quite as well as he deserves, he at least has the unique opportunity of finding three different modes (the paternal, the romantic and the filial) for his one overwhelming passion.

The consistency and intensity of this passion -- Max's never-flagging love for Alice through all of his (and her) permutations -- serve as the anchor for the story, keeping its sometimes baroque flourishes in check. Stylistic excesses and other technical problems are easier to forgive when the emotion behind a novel is strong and sincere, and in the case of ''The Confessions of Max Tivoli,'' such depth of feeling must compensate for a number of sins. The novel's characters, for instance, while convincingly complex and appealing, come off as blithely unhistorical, acting and speaking in ways inappropriate to the novel's late-19th- and early-20th-century setting. (A respectable man and woman meeting for the first time on the street in 1906 would not address each other by their first names; nor would the phrases ''Hi there'' and ''You both look nice'' be likely to crop up in a conversation taking place in 1888.) And although Greer's descriptive passages can be quietly dazzling (I love the peacock ''dragging its gorgeous and filthy ball gown of a tail''), he does sometimes overcook his prose, producing ecstatic arias in which moons explode, breasts glow and brains fill with black stars.

But Max's flights of rapture, if occasionally overripe, are never false, and this emotional honesty (a quality easier to recognize than to define) is what makes the novel memorable. With the character of Max, Greer has achieved a rare balance, creating a figure who is earnest but not humorless, love-smitten without being cloying or tiresome. And when the rhapsodic prose is under control, Max's sense of melancholy can be keenly affecting, as in this passage in which he reflects on the birth of his gerontologically normal sibling:

''Off in her high room, that instant, my sister was being born. . . . Mina was lifted into the world gulping like a lungfish, coughing, and then as the cord was severed and she was made as lonely as any of us, she sang out and Mother could see through the green mist of the chloroform pills that here was her baby. Here was a thing that would grow old; here was a thing that would turn beautiful and lose that beauty, that would inherit the grace but also the bad ear and flawed figure of her mother, that would smile too much and squint too often and spend the last decades of her life creaming away the wrinkles made in youth until she finally gave up and wore a collar of pearls to hide a wattle; here was the ordinary sadness of the world.''

Max's own sadness, on the other hand, is anything but ordinary. But while he may be a monster, he is a profoundly human one, a creature whose unusual disorder, far from making him a freak to be wondered at, simply magnifies his normal and recognizable emotions, sharpening their poignancy. The course of true love, after all, doesn't run smooth -- even for those of us whose biological clocks move forward. So Max turns out to be not so strange a beast after all. He's doomed to improvise his way through life, just like the rest of us, dodging heartbreak and disappointment at every step, forever baffled by the absurd, hopeless ordeal of loving another human being.

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