In quinceañera symbolism, the “last doll” traditionally represents childhood.
The ritual is often called the presentation of the last doll, or última muñeca. A parent or another important relative may give the doll to the quinceañera during the celebration. Its meaning is that she is leaving behind the toys and symbolic childhood associated with earlier years.
The ceremony is related to the broader idea of a transition into a new stage of life. Other customs can express the same transition, including changing from flat shoes to high heels, receiving a ring, or being formally presented to guests.
The last-doll ritual is not required at every quinceañera. Some families omit it, adapt it, or replace it with a different gift. It also does not mean that the honoree is legally an adult; legal adulthood depends on the laws of the country where she lives.