A traditional quinceañera celebrates a girl’s fifteenth birthday, so the answer is 15.
The Spanish word quince means fifteen, and the celebration marks the transition from childhood toward young womanhood. It is especially associated with Latin American communities and with families in the United States who maintain Latin American traditions.
A quinceañera is not simply a birthday party in every family. Many celebrations include a religious service, a formal entrance, a court of honor, special dances, and symbolic gifts. Customs vary by country, region, religion, and family.
The event is sometimes compared with a debut or coming-of-age ceremony, but it is not identical to a legal declaration of adulthood. In most countries, turning fifteen does not itself grant adult legal status. The celebration’s meaning is cultural and ceremonial, and modern families may adapt its elements in many ways.