I have often pondered this, irl to guessing her age which is less explicit and less consistent than most others, changing subtly a number of times and on the whole (IMO) decreases as time goes by. I think EBD is reluctant to allow her to age - hence the increasing obsession with her lack of glasses, perhaps. In Carola she's been teaching for 27 years (which would make her no younger than, what, 48?). In Excitements, she says she was still in her twenties when she joined the school 18/19 years earlier (can't be older than 48?). And then in Challenge she says she was "not much over thirty" when she became headmistress... It's like she's permanently not-quite-fifty.
I sometimes wonder whether Nell's suddenly not-all-white-after-all hair is a similar thing, but I'm not so sure about that. Her age is perfectly consistent (12 years older than Joey, like Madge). I think earlier in the series Hilda is supposed to be noticeably older than this, but later on EBD forgets/prefers to think that all three are much the same age.
Actually not entirely unrelated to this bizarrely-detailed rambling: on mornington's question about book-ending disappointments - my very first was actually the Trials/Theodora 2-in-1 which I think was the final Armada reprint, so I don't recall actual unavailability of titles in that sense, but my access (mostly libraries) was fairly random and limited so I did have that same sense of 'onoes, over again!', if perhaps in a different way. But I am really, really struck reading through now, how frequently I feel saddened actually in the middle of books by the obvious decline in quality, and the good times which are gone and will not come back. I can be sat reading a Swiss one and suddenly feel myself really yearning for the particular innocence of the Tyrol books. Before the Nazis, before Grizel gets her heart trampled over for the umpteenth time, before everyone gets married off and boring - even Daisy the award-winning lady doctor, before Herr Marani dies when Maria is just a carefree Naughty Middle, before Marjorie Durrant loses her husband and child, before Joey becomes so unbearable, before Con Stewart runs off to get married ffs, before Sybil is problematised into a kind of Victorian moral tale, before Evvy Lannis and Hilary Burn lose fiancés in the war...
Agreed mornington and in fact in my not-read-Reunion childhood I was fairly convinced that Grizel and Deira had lived happily ever after in NZ! It would be a better happy ending, but tbh I'm happy to take whatever chance of happiness is offered to Grizel. And I do find Jo quite likeable in how she unhesitatingly opens up her home to Grizel, although it has taken her bloody decades to do so in spite of happily declaring Juliet and Robin 'sisters' all along.