A better questions is how old is Sam?
On May 2, 2012, Sam should be turning 29 - but is he? Just like Dean, Sam's spent a fair bit of time in an alternate "dimension" where time moves differently. And, just like Dean (and everyone in Supernatural, thanks to a continuity error), Sam had two 2010s. Sam might very well be 210 give or take a few years.
Which would actually make Sam the older brother, since Dean might be around 72 or so.
But, that's just their souls, I suppose. Sam's body would in fact still be turning 29 on May 2nd, 2012...or, 30 on May 2nd, 2013 as the case may be. ;)
Timelines. Such fun.
Obligatory Random Sammy Cap: