In second season's Ascension Mrs. Scully tells Mulder that she had given her daughter, Dana, the gold cross necklace that she wears for her fifteenth birthday. In fifth season's Christmas Carol Scully remembers in a flashback that she and Melissa both received a gold cross necklace from their mother for Christmas when Scully was 16.
In first season's Conduit the file Scully reads states that Samantha's full name is Samantha T. Mulder and that her birthday is Jan. 22, 1964. In third season's Paper Clip the file that Mulder finds reads Samantha Ann Mulder and her birthday has changed to Nov. 21, 1965.
In seventh season's Closure Samantha is said to have been taken by the walk-ins when she was 14 in 1979. In ninth season's The Truth Spender states that Samantha died in 1987, which would have made her between 21-23 years old depending on which birthday you go by and what month in 1987 it was.