You are allowed to pay into a pension fund after the age of 75 but you will not receive tax relief on the contributions that you make.
If you are not working, whether a child or a pensioner, you can pay gross £3,600pa. This means that a child would get tax relief, despite being a non-tax payer, i.e. pay net £2,808 with 20% tax relief = £3,600 gross. It also means that a pensioner over age 75 could pay in £3,600 gross with no tax relief.
Whilst the basic tax relief benefit is lost, the pension fund still grows in a tax free environment, much like an ISA, but it does not form part of your estate on death for Inheritance Tax purposes. However, if you pass away after ager 75, your loved ones inherit the pension fund and when they draw down on that fund they would pay income taxes at their normal rates of tax.