It is possible for a child to play in a different age group but there are strict rules that must be adhered to. Please see Regulation 15 Age Grade Rugby on the RFU website for all the details.
https://www.englandrugby.com//dxdam/08/0882fbc2-5069-4b69-b1f6-4dec9bf94385/Regulation%2015.pdf
https://www.englandrugby.com//dxdam/fa/fa1dbfe2-9a2b-4171-9ee8-259313647fc5/AG%20Flyer%202019.pdf
The application has to go through the Club Safeguarding Officer.
Please advise Barney Larkin, Child Safeguarding Officer of any request, before any individual assessment can be made.