The England and Wales Cricket Board has issued guidance about Young Players in Adult Matches. In brief it says:

Clubs must have the explicit written consent of their parents or guardians before young players aged 13 and under as at the previous 31st August can play in adult teams

No young player in the under 11 age group can play in adult teams without explicit written approval of the League or Cup management.

It is strongly recommended that Clubs download the guidance which is available by clicking here.