How do we know that Lizabeth is on the verge of becoming an adult?

From the text:

Joy and rage and wild animal gladness and shame become tangled together in the multicolored skein of fourteen-going-on-fifteen as I recall that devastating moment when I was suddenly more woman than child, years ago in Miss Lottie’s yard.

By the time I was fourteen, my brother Joey and I were the only children left at our house, the older ones having left home for early marriage or the lure of the city, and the two babies having been sent to relatives who might care for them better than we.

And I remember, that year, a strange restlessness of body and of spirit, a feeling that something old and familiar was ending, and something unknown and therefore terrifying was beginning.