@Jasmine: She most likely would have been young or possibly adult with a youthful appearance. I’ve actually seen some actors and actresses in movies that look younger than they really are. But that’s kind of beside the point. So, it’s possible the White Witch may have been an adult with youthful appearance. 

Normally, in Western society, we judge who is a child and who is an adult by legal age. I reached the age of adulthood, in 1969, at a time when the age of majority was 21, enabling me to vote, marry without asking my parents or be otherwise treated as an adult. This age was reduced to the age of 18 years about when my eldest child was born in 1972. You say that some actors & actresses in movies look younger than they really are, a very good point, when the ability to disguise age is part of their professional work, and there are also people, I agree, who work very hard at staying younger than their actual age. In her only other book appearance in the Narnia Chronicles, Polly says of Susan:

"Grown-up, indeed," said the Lady Polly. "I wish she would grow up. She wasted all her school time wanting to be the age she is now, and she'll waste all the rest of her life trying to stay that age. Her whole idea is to race on to the silliest time of one's life as quick as she can and then stop there as long as she can." 

We never find out what sort of actual work Susan would likely be doing, once she left school. C.S.Lewis did write to someone about Susan that though she was proud and vain in The Last Battle, she would have time to change in her own way, when she was the only one of the Pevensies, who remained alive. 

Whether for career or social reasons or whether it is a natural lifestyle choice, many do try to keep looking as young as they can, until well past middle age, which is roughly in the 50's age bracket, when certain physiological changes can & do take place, along with that particular milestone. One or two decades later, retiring age is the next milestone, at the traditional three score & ten, though some still can work well into their eighties & nineties. But by that time health can also go downhill fast. It is cruel of thoughtless people to call elderly women "old hags", or "old witches", either, in my opinion, when elderly women are often too fragile to be anything of the sort.

Jadis, I think, goes into this category, of staying at the same age for the rest of her life, sort of "mutton dressed up as lamb", as the likes of Aunt Letty would say rather witheringly. Usually, it is done with paint of one kind or the other, whether as cosmetics or in the case of statues, actual paint. But it seems that Jadis was skilled with magic. Therefore, I don't think Jadis' actual age is relevant to the plot when Jadis wants to stay at the same age forever, anyway.