Can a junior play in an adult match I'm umpiring, and what do I need to know?

Under the ECB Recreational Cricket Safety Regulations:

• Juniors can play open age cricket from age 12 (11 only in exceptional, approved cases), with written parental consent.
• The captain must tell the umpires about any juniors in their team.
• A junior keeps their own age's safety rules in an adult match: fast bowling limits, minimum fielding distances and helmet requirements all still apply.

In ECB competitions, each side must give the umpires a team sheet before the match identifying every player under 18 and showing the age of any player aged 19 or younger, so you can apply the fast bowling and fielding rules.

If a captain suspects an opposing player is too young for open age cricket, the Regulations say the captains should discuss it, and if still not satisfied, the captain reports it to the competition organiser.