I'd estimate that the correct answer may be as low as 0.1%, though if we only look at this question: is it possible to be convicted of a Criminal Offence for simply travelling without a ticket, with the wrong ticket,or with the correct ticket but without a Railcard, then perhaps a massive 1 or 2% would be correct.
Are the railways being unique as the only non-police/CPS body who criminally prosecute (ignoring the odd private prosecution such as Stephen Lawrence, etc)?
If I didn't enjoy my meal in a restaurant I don't pay. And I don't pay safe in the knowledge that the waiter can't whip a pad out and write an MG11 out.
If I put something in my Tesco basket which I later change my mind about and remove it, I remove it safe in the knowledge that the store manager can't prosecute me in the magistrates court (or any court) - including even if I have read the magazine or newspaper.
If I read a newspaper of the guy next to me in the park bench, I am secure that he can't suddenly take me to court for not paying to read half his paper.
Why are the railways different? The answer I believe is a mixture of their archaic rules, the lack of a tangible product, and the industry's reluctance to provide the framework for 100% tickets bought before entering barriers.
Regarding the OP, I wouldn't claim to be familiar with the rules in Wales, however here (West Yorkshire) child tickets are until 31st Aug after the 16th birthday (as child tickets require a Metro half fare permit to purchase them). Others can comment on wales or other areas.