I had good fun when I was 14/15...
I looked (and still do 2/3 years later) a fair bit older than I am. It was horrendous at times, if I wasn't travelling locally (where a lot of guards either know me or can't be bothered going through the train) then I could spend days literally being challenged every half an hour or so.
I used the train to school so had a child rate photocard which I carried at all times (and used pretty much 363 days a year!) So never actually got made to pay full fare-but let me put it this way, I got so sick of being accused of being a liar every other day, I actually looked into getting a 16-25 railcard BEFORE I turned 16! And yes, the reactions from staff varied between 'how old are you please?' To a point blank 'You're lying, you're definately over 16 mate'
My absolute favourite argument though was in the Cardiff Valleys:
Guard: 'Tickets please'
Hands over a child Valley Lines Explorer
Guard: 'How old are you...?'
Me: '15'
Guard: 'Hmm... Have you got any ID please?'
Pulls out season ticket and shows the photocard
Me: 'Yep, here you go'
Guard: (direct quote) 'But having that doesn't prove you're under 16 though does it?'
No words. Literally no words...
I also had a couple of arguments with staff who, upon finding they couldn't do me for my age, would try and find anything and everything else to argue with!
And more recently I had a bus driver the other day question me pulling out an under 19s bus ticket. He asked how old I was, I said 17 and offered to show ID but he just laughed and said it was fine.
On the subject of Euston barrier staff, they even like to be difficult with other rail staff! I know of someone who arrived into London earlier than planned and faced a lot of hassle trying to get through to have a word with the guard. This was for the very last train of the night too... if they'll have attitude with other TOC's staff then us passengers have no chance!