Every cinema will class the age of a child slightly differently, these are listed below and also made clear on the voucher page before purchase.
Cineworld: 14 years or under
Empire: 14 years or under
Merlin: 14 years or under
Movie House: 12 years or under
Movies@Cinemas: 14 years or under
ODEON: 12 years or under
Picturehouse: 14 years or under
Reel Cinemas: 12 years or under
Scott Cinemas: 14 years or under
Showcase: 14 years or under
The Light Cinemas: 17 years or under