It sounds as if a ticket inspector accused you of travelling with an invalid ticket, in that you bought a child ticket when (in railway terms) you needed an adult one. Is that right? If the inspector took your name and address and other details to send a report to his company, then you haven’t been issued a penalty fare (the railway equivalent of an on the spot fine). Anything you know or read about penalty fares is not relevant to your situation and you will confuse yourself if you think about them.
Instead, the train company will be considering what evidence there is of you committing offences under the various railway laws. At some point in the next few weeks or months they are likely to write to you, or perhaps to your parent or guardian. When they do that, I suggest you post a copy of the letter here (making sure you don’t post your name, address, or anything that identifies your ticket, the case number, etc) and people here can give some advice. They may want to try to investigate how many journeys you’ve made with invalid tickets, and they will want at least payment for those journeys and a meaningful contribution to the cost of investigating, and they are likely to threaten prosecution (although railway companies prosecuting people under 18 is complex so less common than prosecution of adults).
