In the Superman episode "Monkey Fun", which is set in 1997, it is revealed through flashbacks and dialogue that twenty years before the episode, in 1977 Lois was 8 years old. It follows that Lois would have been born 8 years previously in 1969, and that Lois is 27 when she's first introduced in "The Last Son of Krypton, Part II", set in 1996, and 28 years old in the present day events of "Monkey Fun".