Sanskar
Vedarambha
The formal start of Veda, or of the family's principal text.
When
After upanayana, where that tradition exists.
Vedarambha makes study a formal sankalp. In one house it is a Vedic hymn, in another the first chapter of the Gita, in another the principal text of the shakha. The name says Veda; the practice follows the family.
Modern schooling is long and different. This rite does not replace it. It remembers that learning is a debt, not only an examination.
This is a general introduction. Sequence, materials, and timing change with family, region, and shakha. Your priest and your own custom have the last word.