I just assumed that Lisa would have asked him at some point, like maybe if he arrived in time for Ben's birthday, or if Lisa's birthday was late in the year - she would have realized she didn't know when Dean's birthday was and asked...

But you make a good point. All evidence suggests that the Winchesters don't really celebrate birthdays, so it might very well be that he just never told them... which would make that year even more depressing. :(

HUGS FOR DEAN!