There are various variations for each age group. See below a link to the RFU Regulations with Regulation 15 specifically on age grade differences.
This will inform you about:
•Regulations - For each age group, length of matches, size of pitch, size of ball etc
•Laws - modifications for each age group to IRB laws (ie what the ref uses at your age group!)
•Guidelines - to assist in the understanding, interpretation and implementation of Age Grade Regulations
RFU Rules and Regulations