Yes, it's particularly odd in that she's absolutely babied by everyone at the school from the moment she arrives! There's that moment when they pull back the blanket (Miss Maynard must have the strength of an ox, as she appears to have carried a sleeping six year old in her arms the entire way from Spartz to the ferry landing!) and it's all 'such a lovely baby face', and bathing her in a baby bath in the dormitory, as though she's a toddler at most.
I mean, Amy Stevens was only eight when she started at the CS in its first week or two of existence, and no one seems to have been babying her to anywhere near the same extent.
EBD is fascinated by children who have lived exclusively with adults, but she has some deeply inconsistent strategies for explaining their behaviour in relation to those adults. You have the Robin, who behaves as though she is much younger than her age until she's into her teens. (How old is she when they flee Austria in Exile? -- because people are still carrying her on their shoulders like a doll!)
You have Polly Heriot, who is dying to be a madcap schoolgirl after an upbringing by elderly guardians.
You have Verity, who is prim and proper and disapproves of modern children, after being brought up by two elderly people and a governess who was in her 70s.
You have Eustacia (whose parents are older as well as a scholar and a medic), who is also prim and proper, but in a scholarly way.
And maybe rather more oddly, you have Mary-Lou, essentially brought up by her grandmother, as her mother is a wet lettuce, who apparently was a 'quiet little puss' before meeting Clem and Tony, after which she turns into a tomboy but still 'doesn't like girls'.