You still pay adult fare from age 14 on (Arriva) buses in Northumberland and North Yorkshire.Not sure there was a particular connection between child fares and school ages - except that scholars could get seasons till they turned 18 (I did), and group travel for under 18s was at half fare (I booked some).
The uniform child age across much of Europe was 12 (maybe still is in places) and in Germany it is 15 now.