Keśānta
The popular claimThe first shave is the only saṁskāra in the list that marks the body of an adolescent, and it is the one that has vanished most completely.
Thirteenth in the list, at about the sixteenth year: the first time a razor is taken to a young man's face.
The rite has two names and they point at two different things in it. Keśānta is "the end of the hair" and describes the act. Godāna is "the gift of a cow" and describes what is given — to the teacher, by the student's family, on the day. Which name a text uses tells you what that text thought the occasion was for.
What is done
The procedure is openly borrowed from Cūḍākaraṇa, the child's tonsure, eleven years earlier — the same warm water, the same darbha blades laid in the hair, the same order of the head, the same verses adapted. A tradition that wanted to make a point about repetition could hardly have made it more plainly: the body is marked the same way at the start of childhood and at its end.
The beard is cut, and in most versions the head hair and the body hair too. The student is bathed and given fresh clothes. The cow is given to the teacher. And then the part that the sūtras care most about: the young man takes a vow.
The vow is the rite
For a year after Keśānta the student undertakes restraint — celibacy, and in the stricter formulations a set of further observances: no honey, no meat, sleeping on the ground, no daytime sleep. The texts spend more words on this year than on the shaving.
Read in sequence the logic is not hard to follow. The rite is placed at the age when a boy's body becomes capable of fathering a child, it acknowledges that fact publicly by cutting the hair that announces it, and it then asks for a year of deliberate restraint before anything is done about it. Whatever one thinks of the prescription, it is a coherent response to the thing it is responding to, and it is notably unembarrassed.
The age, and the disagreement about it
Sixteen is the usual figure and Manu gives it. But the sources vary by varṇa in the same way they vary for the age of Upanayana, and the variation is mechanical: the ages are staggered, and Keśānta moves with them. Some authorities place it at eighteen, some at twenty-two.
This is one of several places in the sixteen where the system's stratification is visible rather than implied. We are not going to pretend it is not there.
Why nothing replaced it
Keśānta is effectively extinct. There is no widely observed Hindu ceremony for a boy's first shave, and unlike Puṁsavana — which disappeared and left the pregnancy celebrated by a different rite — this one left a gap that stayed a gap.
Two reasons suggest themselves and we offer them as observation rather than as scholarship. The rite was tied to the studentship it regulated, and when formal brahmacarya ceased to be the ordinary shape of adolescence the vow had nothing to attach to. And a ceremony about a boy's sexual maturity, performed in front of relatives, is the kind of thing that families quietly stop doing.
The gift of the cow survived it. Godāna as a form of dāna continued long after the occasion that carried it was forgotten, which is a pattern worth noticing across this whole section: the giving outlasts the rite.
What this page does not say
It does not recommend the year of restraint to anybody, and it does not present the staggered ages by varṇa as anything other than what the texts say. Reporting a rule is not agreeing with it.
It does not say that a young man who shaved for the first time on an ordinary morning, with no cow and no verse, has missed a necessary step in becoming an adult. He has not. The rite named a real transition; the transition happens whether or not anyone names it.
Source — Ashvalayana Grhya Sutra 1.18 and Paraskara Grhya Sutra 2.1 on Godana/Kesanta; Manusmriti 2.65 on the age; the Chudakarana procedure from which the rite borrows its form
Every claim on this site names its source, and every calculator shows its arithmetic.