They specifically had Sam say, just a day before the deal was made, that he was still 23 years old. (Along with all the other specials in Ghost Town, USA.) So at best, he has just turned 24, and he cannot turn 25 before the deal runs out.
OK, let's all pretend that I actually know math. You're right, Nutty, Sam is currently 24. I calculated Sam's age based on 2008 accidentally, not on 2007.
Dean, however, is 28, because he is four months older than me. Thankfully I didn't fuck up my own age.