Since essentially DS was based in the Gothic tradition, it does make sense that there should be young and innocent heroines and dark and brooding, older heroes. That's what the Gothic (and no, I do not mean of the paperback variety, although it is related to the original) was all about. The heroine's parents are typically non-existent or dead. The mother never seems to have been significant; the father is mentioned more. Young woman from impoverished or persecuted background goes to old dark house where she encounters villainous older/other women (usually) and the Byronic sinister-seeming older man. The Gothic hero serves as a substitute father figure (with all the Freudian undertones you can pack into that one a given). Tension, violence, death, supernatural usually a plus. Gothic hero has possibility of being wounded/maimed/taken down several notches (e.g. Mr. Rochester). Villainous older woman dies a well-deserved death. Heroine learns all about the house and the hero's deep, dark secrets. She grows up. Gothic hero and heroine marry and here's the important thing: "Girl gets house" (quoting Abby Adams Westlake).
Okay, so when the show made its debut, Vicki is perfect Gothic material: orphan, mysterious origin, young and sweet and innocent. Burke served as the Byronic hero. Laura and to some extent Elizabeth and Carolyn filled in for the older/other woman type thing.
But it's not really a pure Gothic program and the show morphed into other genres. Vicki never really loses her innocence. She never grows up. More importantly, she stops being a heroine. Vicki stops being the focus of the show. The SYT girls that come along never achieve heroine status.
After Vicki becomes a secondary character, all of the SYTs are, well they're just sort of there. They're objectified. They're objects of desire or hatred, but they cease to be the ones actually uncovering mysteries or causing the action. They become reactive rather than active.
Rachel is an unfortunate attempt on the part of the writers to recapture the Gothic tradition, but the character is written as if she's a nitwit rather than a heroine. Here's an example that I've used before:
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Gothic heroine: Vicki comes to Collinwood and immediately encounters what we would now term as one hostile working environment. Vicki is told to stay out of the basement of Collinwood. Late at night she hears mysterious sobbing. Being a nice girl and wanting to help whoever is in trouble, she gets up in the dark night and goes poking around and ends up in the basement. What does this do?
1. Establishes that Vicki is a caring, adventurous, young woman worthy of our attention and audience identification.
2. Provides a nice supernatural touch.
3. Moves the plot along.
Gothic moron: Rachel arrives at Collinwood to be governess and also immediately encounters the hostile working environment. She sees some lights in a closed-off portion of the house, inquires and is told emphatically by the owner of the house, "There are no lights. Don't go to the tower room." Even though it is none of her business and even though no one appears to be in any danger in the tower room (she just saw some lights), she goes traipsing off to the tower room. What does this do?
1. Unintentionally establishes that Rachel is a nitwit. Most of us would not jeopardize our first day on the job with the nasty scary employers by poking around without justification.
2. Moves the plot along.
Vicki is a heroine. Rachel is window-dressing. None of the SYTs change except possibly to grow more innocent, and more naive. They don't grow up. Forever Peter Pan as it were.
As for Barnabas. You can argue till the cows come home that he is meant to be a young man, and that's fine, but youth doesn't necessarily equate to innocence or naivete or being drawn in turn to those qualities. Carolyn is supposed to be younger than Vicki, but she never comes across as a SYT; she's too knowing for that.
Luciaphil