Is it really a train company's failing if they can't do the impossible and age someone on sight?
As you say, it is impossible to age someone on sight. However, in the instances reported on this thread (and elsewhere) TOC employees are making categorical assertions regarding someone's age. In some cases the TOC employee has falsely stated in writing that someone is not a child. Such behaviour shows, at best, a lack of training which itself is a failure of the TOC. If, even after appropriate (re)training, a employee continues to falsely accuse people of criminal behaviour based on a guess of age then the failure of the TOC to deal with the matter is an even bigger issue.