The Alice books celebrate youth as a time when the individual is open to imaginative possibilities. Childhood is praised, not exactly as a period of innocence, but as a state in which many things are possible. Children can't help growing up, but they can refuse to grow old, and even old men can maintain a youthful outlook by preserving a spirit of nonsense and adventure. One can be either too young or too old, and the best course seems to be digging in one's heels and insisting on remaining as childlike as possible. Growth is depicted as out of one's control, but emotional growth can, perhaps, be resisted. Adulthood in these books seems almost ridiculous in contrast with youth; adults are bossy know-it-alls who like to throw their weight around and rain on the parades of the young.
Alice's youth is the source of her imaginative capability, and the books suggest that, as she ages, her ability to conjure up fantasy worlds will diminish.
Alice grows and changes in a variety of ways in Wonderland and Looking-Glass World, but her childlike nature doesn't depend on her size or age.