Complex sentence: Tabitha was born in 1966 and should have been 11-years-old in 1977, a fact that would not have worked with the major theme of the show about young woman out to make it on her own.
Split into simpler sentences: Tabitha was born in 1966 and should have been 11-years-old in 1977. A premise that would not have worked with a major theme of the show, a young woman out to make it on her own.