Thelma Evans, played by the late, great Bern Nadette Stanis, was 22 years old when the show premiered in 1974. Thelma's age was never explicitly stated in the series, but her character arc and interactions with other characters suggest she was a young adult in her early twenties.
Thelma's character was meant to be a relatable and aspirational young woman navigating life, love, and career in a changing America. Her age reflected the demographic of young adults who were experiencing the social and cultural shifts of the 1970s.
While Thelma's actual age remained a bit of a mystery, her role in the show and her youthful energy made her a beloved character for generations of viewers.