sam would have to be at least mid 30's. she was in the gulf war, which was 1990 and at the very, very youngest would be 18 but likely early 20's given her doctorate etc.
add 11 years and it puts her right in the vicinity of early-mid 30's.
also she was promoted to major two years ago and i believe that you have to be in the air force at least 12 years before you can be a major so this also works out to mid 30's
as to other characters b-days....
in the light teal'c says he will be 102 in 47 days, and generally speaking the day the show airs on showtime is the date it happens so his birthday is march 14th i think(i'd have to go look for sure)
and in FIAD danny says his b-day is july 8 then in 1969 he says he's 4 1/2 so it puts him born 1964-65 which makes him roughly 36 so.....technically sam is the baby of the group. either that or she and danny are really close in age, like less than 5 years difference.
the only mention of jack's age is in brief candle when he makes a comment like 'i'm only 40 and i look 80' or something like that. he makes a reference to either being 40 or in his 40's