*finally sees this*
Oh, that's distressing thought!
But that's based on the assumption that Nyssa was the same age as Sarah Sutton in
Terminus but NOT the same age as Sarah Sutton in
Castrovalva, in order to accommodate the additonal years inserted by Big Finish. Also, you're assuming that Tegan's guess about Nyssa's age in
Cobwebs is correct.
Since that doesn't work, I suggest starting with a different set of assumptions?
Namely, that Nyssa was the same age as Sarah Sutton in
Castrovalva, and older than she looked in
Terminus. Tegan would be none the wiser, if Nyssa never told her how long she traveled with the Doctor while Tegan was back on Earth.
I've got a few reasons for thinking that.
For one thing, Nyssa's approximately Ann Talbot's age in
Black Orchid, which means she's old enough to get married before the end of Season 19. (
Black Orchid was filmed in October 1981, two months shy of Sarah's 20th birthday.) By
Time-Flight, Sarah was 20 years old.
For another, when Tegan says "you look pretty good for seventy," Nyssa immediately
corrects her, reminding Tegan (as if they've had this conversation before) that she's older than she looks. Similarly, in
The Elite, Tegan complains that Nyssa shouldn't have forgotten where they stashed her belongings, because it's only been a few months since she left, but Nyssa corrects her: "For you, maybe, but for us it's been a little longer." Which is almost certainly a tactful understatement.
(In fact, Nyssa's probably not sure any more, unless the TARDIS keeps a record of relative time since she came aboard.)
Here's my
Nyssa Timeline. Big Finish has inserted at least 2 years between Season 19 and 20; probably rather more since I'm only counting days/nights that are explicitly and definitively mentioned (for example,
Black Orchid probably covered more than one day, since the Doctor & co stay for the funeral, but I only counted it as one day on my chart because we didn't see a definitive "night" pass after their arrival).
Then we come to the audio
Circular Time. Enough time has passed between "Spring," "Summer," and "Autumn" for Nyssa's relationship with the Doctor to have changed: from ward and guardian, to partners, to old friends who are beginning to lead separate lives from one another. That transition has to cover a span of years, not months. More importantly,
the writer of "Autumn" is clearly portraying Nyssa as a young woman, not a child. The Doctor respects her decisions about Andrew (even if he thinks 'that boy' is a tosser) because, in the Doctor's eyes, Nyssa is an adult and not a minor. I think we should trust the Doctor and the scriptwriter on that point.
If we have to adjust Nyssa's age to accommodate Big Finish, it makes far more sense to move her numerical age up at the end of her journey rather than lowering it at the start.
Cobwebs provides the perfect excuse: She doesn't age at the same rate as humans, so 70something Nyssa looks like 50something Sarah.
Okay, enough geeking.
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Apologies for not replying sooner!
My enthusiasm for DA was dampened by negative feedback.
Occasionally, I get the urge to draw shippy stuff when I'm depressed and need to cheer myself up with fluff. Yet I'm well aware people don't like my ship, and I feel rather self-conscious about it. The last time I broke down and indulged in my guilty pleasure, I hid it in Scraps where those who consider it a NOTP would hopefully not see it on their dash. No such luck: almost immediately I received a comment informing me that the commenter didn't like the ship. It's hard for me to work up enthusiasm for posting fanart when the thing I enjoy drawing most will be ignored, at best, or at worst, will attract "I don't approve of what gives you joy."
(Which is silly, since others
do enjoy that ship. But unfortunately I've got anxiety/depression that tends to amplify negative feedback and minimize positive feedback. It's my problem, not anyone else's, but it tends to dictate where I'm most active.)
So I pulled back from DA. It wasn't just the shipping issue, but that was the straw that broke the camel's back.
Instead, I'm invested in fandom communities and maintain four busy blogs on Tumblr. I've got two semi-dormant DW blogs and two semi-professional blogs (one of them that VHSWhovian review website). And, as you know, I write fanfic, although I sink so much time and effort into planning, rewriting, and polishing that every chapter takes months.
And that's quite apart from real life, where chronic illness cuts into my time and energy significantly. Since I can't be active everywhere, DA tends to be ones of the places (like DW) that get shortchanged. Sorry about that!