1)All information comes from my own timelines.
2)I use chronological time, rather than the "soap-opera" time that the show uses.
1973 - John Winchester proposes to Mary Campbell...and then Mary makes a deal with a Yellow-Eyed demon to save John's life.
~May 1978 - The angel Anna attempts to kill Mary. Two strangers from the future save her, and then claim to be her sons. She then gets her memories erased. She's already pregnant with Dean.
1983 - Sam Winchester is born! And he is ADORABLE!
2006 - Having just successfully saved his brother from heart failure, Sam is dragged to Missouri (the state) to deal with a racist killer truck. Sam turns 23.
2007 - Sam is resurrected from the dead on his 24th birthday...and he is still ADORABLE! Dean hugs him. Sadly, Dean sold his soul to bring him back, and Sam isn't very happy about that.
2008 - Dean is torn apart by Hell Hounds while Sam watches helplessly. As far as birthday's go, I'm willing to bet Sam's 25th birthday is the worst.
2009 - Sam tries to keep Jimmy Novak safe after Castiel gets dragged out of his vessel. Jimmy gives Sam the slip while Sam is feeding his demon-blood addiction...it's really all downhill from there. Sam turns 26.
2010 - We never find out when Swan Song takes place, but by my calculations, it was probably very close to Sam's 27th birthday. Perhaps Sam and Dean had one last chance to celebrate before Sam sacrificed himself. Perhaps it was already too late by the time Sam's birthday rolled around, and Dean raised a glass to his brother all alone in Cicero, Indiana....or perhaps the day got lost in the swirl of planning, and demon killing, and final goodbyes.
2011 - Similarly, we don't know when Dean discovered Sam's return. But in 2011, it's safe to say that Sam's body was topside, hunting with Samuel and perhaps seeing Dean for the first time in a long year. Meanwhile, Sam's soul was still suffering. Sam is 28.
2012 - A fully restored Sam and Dean celebrate Sam's 29th birthday with Bobby in South Dakota after the events of My Heart Will Go On.
2013 - Sam and Dean bid farewell to Charlie Bradbury after successfully pulling off a heist. Sam turns 30, while they try to figure out what's so special about a suitcase full of clay.
2014 - Amelia surprises Sam with a birthday picnic in the park for Sam's 31st birthday. Sam isn't quite sure how to react. Meanwhile, Dean is wandering around Purgatory with Cas and Benny trying to find the exit.
2015 - ???
HAPPY BIRTHDAY SAM!!