What you guys say is quite interesting, but I think Derek is about the same age as Mark, isn't he ? (as they grew up together), when Mark died it was indicated he was 44 years old.
I didn't know that the first 3 seasons were supposed to have happened only on 1 year (Grey's Anatomy time), do you guys say that cause they passed their intern exam in season 3 ?
Then it means that Callie and Baily were 5th year resident when the others arrived (as Callie/Bailey were both chief residents at the end of season 3/beginning of season 4.
Lexie said even that she was 25 years old and she had skipped 4th grade or something like that, when Mark started dating her.