You can indeed claim a deduction under Section 80C for contributions made to your major son's Public Provident Fund (PPF) account. The law specifies 'any child' and does not distinguish based on age. Therefore, as the depositor, you remain eligible for the tax deduction regardless of your son's majority status.
13 February 2020
The words used in the Act [section 80C(4) ] is "Any child of such individual" The Act is silent on the issue of majority or minority of child.
14 February 2020
Correctly stated by Amol Sir. Deduction would be available even if deposited in PPF account of major son also. The depositor would be eligible for deduction.