Who has it today
Historically the texts prescribed Upanayana for boys of the brāhmaṇa, kṣatriya and vaiśya communities, at different ages. Today it is mainly kept by brāhmaṇa families and some other communities, often shortly before marriage if not done in childhood.
Practice has also widened. Some ancient texts mention girls receiving it, and in modern times the Arya Samaj and certain other organisations and families perform Upanayana for girls and for people of any background. Families follow their own tradition; all approaches are described here neutrally.