I'm pretty sure Sheppard's birth date was never pinned down in canon. Rodney's birthdate was used for a password once, so we know what it was (1968) but I'm pretty sure there is nothing likewise for any of the other characters (and I've hunted for it!). The general fannish consensus seems to be to use the actor's age, but I don't see any reason why you'd have to (and in fact there are a number of cases in which the character seems evidently a different age than the actor playing them, like Keller, or -- canonically -- Teal'c ... in fact, come to think of it, if Cam was supposed to be 38 in 2007, then the character is several years younger than the actor).