I've just noticed that Centrebus use an age limit of under 14 for child tickets with ID and under 11 for those without unless a "special local scheme operates."
The special schemes seem to be:
Hertfordshire: 11-15 if the child holds a Savercard
Derbyshire: Child rate up to 14 and reduced rate up to 18 with B Line card.
West Yorkshire: Under 16 or under 18 if in full time education with photocard.
http://www.centrebus.info/Pages/YoungPersons.aspx
That seems a lot of variation for an operator that's not that huge. It seems like they've taken the Derbyshire under 14 and implemented it nationwide without implementing the reduced rate for older children still in full time education nationwide.
The special schemes seem to be:
Hertfordshire: 11-15 if the child holds a Savercard
Derbyshire: Child rate up to 14 and reduced rate up to 18 with B Line card.
West Yorkshire: Under 16 or under 18 if in full time education with photocard.
http://www.centrebus.info/Pages/YoungPersons.aspx
That seems a lot of variation for an operator that's not that huge. It seems like they've taken the Derbyshire under 14 and implemented it nationwide without implementing the reduced rate for older children still in full time education nationwide.