I had this and because my son is now 10, and this is not included in the current season ticket, he now classed as Under 18 bit naughty.
Indeed, I asked about this at our presentation and was told he would pay the same rate as his st should there be any cup or playoff games after his birthday! Rang up and they changed it to u16 but online tried to charge me adult price for him so rang ticket office second time and they changed it back to u18 and told me to take my issue up with customer services if I'm "that bothered about £8"