Brad Pitt's character, Benjamin, was born an old man and promptly abandoned by his father.  He was raised by a caretaker of an old folks home in New Orleans.  This was the perfect place for him, an old folks home, since he's already an old man.  LOL  As he gets to be in his teens (and roughly in his 70s/60s) Benjamin moves out on his own and goes to work on a tug boat.  While on his travels, he writes to one of his friends back home, Daisy.  She was the granddaughter of one of the women in the old folks home and she always knew there was something different about Benjamin.  So Benjamin eventually comes home and Daisy is now a young woman and he's still an old man (60s/50s).  Daisy is also a ballerina and after a failed attempt at seducing Benjamin, she goes overseas where she's in a terrible accident.  When Benjamin goes to see her, she tells him she doesn't want him to see her like she is (stuck in a bed, very cut up) and so he leaves.  Daisy eventually goes looking for Benjamin once she is healed and they get together.  I love this part of the movie because their ages aren't too far apart at this point and they make sweet sweet love all over the place.  LOL  

So imagine you are with a woman who gets older and you get younger.  Imagine trying to be a father to a child that gets older as you get younger.  These what ifs plague Benjamin and create an interesting scenario to ponder.  

The acting is superb, Brad Pitt is beautiful to look at.  The movie does run over 2 hours, so get comfortable.  It's good stuff though.