India, Aug. 21 -- This August, the first children of free India turn 79; lakhs more are older than India's independence itself. Lakshmi, born in the monsoon of 1947, has lived the entire arc of independent India, from ration cards to UPI. She now inhabits a country transformed in every respect but one: Its laws still assume that a joint family will catch her when she falls. That household, in much of India, no longer exists in the form the law imagines.

The Sample Registration System's latest report puts India's total fertility rate at 1.9, i.e., below replacement levels. India has roughly 150 million people over 60 years of age; by 2050, that number will touch 347 million. And the transition is geographically uneven. The states that led...