Jack Nicholson is like oxygen: easy to take for granted.

With his trademark grin, lupine air and irrepressible rebelliousness, he is an icon whose talent is too often overshadowed by his status.

As ageing retiree Warren Schmidt, however, Nicholson reins in the Jackisms. He trades the slickback hair for a comb-over and transforms from legendary lothario into fragile old man. It's a magnificent performance.

And Schmidt is a magnificent character. We join him clockwatching in an empty office, as the seconds tick away on his last day in the "insurance game".

A buttoned-down drone who once dreamed of entrepreneurial glory, he's irritated by his condescending replacement and can't bear staying at home with his prissy wife.

Desperate for meaning, he sets out on a road trip to Denver, where he hopes to dissuade his daughter Jeannie (Hope Davis) from marrying sweet-but-dumb waterbed salesman Randall (Dermot Mulroney).

Schmidt loathes this ponytailed idiot, and he's certainly not hard to smirk at. However, for all the jokes that are had at the expense of Randall and his hippy mother (Kathy Bates), Warren is just as amusing and tragic.

His verbal impotence and self-doubt are made all the more affecting because it is Nicholson who displays them, a star who usually appears charismatic, witty and sexually confident. Here, he's bathed in sadness, striving to find himself. This is a coming of age flick for the elderly.

As with "Election", director/co-writer Alexander Payne astutely dissects the life of a dissatisfied American male, satirising both conservative and pseudo-alternative values. But while it's easy to sneer at the protagonists, there's heart here as well as laughs.

The audience may be encouraged to share Warren's guffaw-full prejudices, but that simply makes us culpable when it emerges that this bitter pensioner is only now realising the importance of unconditional love.

This is a startling, resonant, life-affirming movie - both deeply funny and deeply moving. And, like Nicholson, it's only likely to improve with age.