sorry nat67 can you look at your post and edit it a bit i'm not following it all
Neither am I and I was there when it happened
This is what happened (as Nat seemingly wants to say).
The two of us were making a day return from Derby to Tamworth using a family and friends railcard. Before boarding the train from Derby to Tamworth (a Cross Country 170) I asked the guard if he could sell us a ticket on board as the train was due to leave in 2 minutes and we hadn't yet purchased one to Tamworth. He was fine with this and sold it to us on board and we had no further trouble on the ride to Tamworth, where a short wait was had for the 40 minutes late 12:25 Plymouth-Glasgow service ('1S51').
This service was busy so we ended up stood in the vestibule, me in one and Nat67 in the one next to it (so two separate coaches). The guard got to me first and I showed him my ticket. He noticed that I held the child part of a family and friends discounted ticket yet also had my other railcard on view so got confused and asked if I was using that, to which I replied no and said that I was using the family and friends railcard that Nat held (who had come into the same vestibule as me at this point). He asked how old I was and I replied with 16, leading to him saying that I couldn't use the family and friends railcard if I was 16. I told him that the family and friends railcard T&Cs clearly state that a family and friends railcard can be used until the child turns 17 if the railcard was bought before they turn 16 (which he didn't query oddly enough).
He then turned to Nat and apparently didn't believe that he was the railcard holder until Nat showed him his YP railcard and proved that the names matched (why you'd question that I don't know). He then asked when I turned 17 which I replied to truthfully and then asked if I had any ID (I didn't) before saying that it's compulsory to carry ID when travelling on a child ticket. I told him it wasn't and he then just left.
Sorry about that essay but I hope it shows what Nat67 was trying to say a little better.