Hi,
The criteria to pay Voluntary National Insurance Contributions (NICs) depends on an individuals circumstance, however I can provide some general information on this matter.
You can only pay voluntary NICs from the tax year you are registered on the NI system, you cannot pay prior to this year. For example you register for a UK NI number in May 2024 you can only pay Voluntary NICs from the 06 April 2024. You cannot pay Voluntary NICs in the year you reach UK state pension age.
There is more information held on the
GOV.UK website
Pay voluntary Class 3 National Insurance
Thank you.