How old must you be to stay at Splash Landings hotel in Alton Towers?

My sister wants to stay with a few of her friends for one night at Splash Landings hotel... but they are all 15, with the oldest being 16.

Surely if their fares are charged at adult, from the age of twelve, they should be able to stay at a hotel as adults.

Could you please help, thankyou.

the website says: An adult is regarded as a responsible adult over the age of 16.

http://www.altontowers.com/pages/prices