Young persons, and I think HM Forces railcards have a minimum fares rule, which states that, whilst the railcard can be used before 10:00, there is a minimum fare of £12 on all walk up tickets which obviously means that for many short distance journeys the railcard effectively can't be used before 10:00. In the case of YP railcards, this rule does not apply in July or August. I am not sure on the restriction for advance tickets or rovers. Many people wrongly believe that the railcard can't be used at all before 10:00. My interpretation is if you buy your ticket before midnight, and it would ordinarily be valid until 02:30, then you are valid with your discount, but if you buy after midnight then the minimum fare rule applies. Also, Network Railcards and Network Gold cards are not valid at all before 10:00 on weekdays.