Although both represent the coming of age of a girl, they are celebrated in two cultures and at different ages.
Sweet 16s are typically held in Northern America (and other places as well) and are parties held to celebrate a girl turning 16. It's usually more festive and meant to be a fun party, with loose customs.
Quinceneras are mainly celebrated in Latin America and celebrates the transition of a girl, from childhood to womanhood, at the age of 15. It is a very important tradition that includes a ceremony (then an afterward party), with families saving money even before the birth of their daughter! There are many customs that are upheld, such as the changing of the shoes, when the birthday girl wears flats in the beginning of the ceremony then later changes to heels, symbolic of her entering womanhood.
