Thanks for confirming.
As you're probably now aware travelling with a child ticket when you're 17 is a criminal offence. Transpennine Express are one of the more pragmatic train companies when it comes to dealing with this sort of thing. It should be sortable but it will cost you a bit of time, effort and money. If you haven't already done so, I suggest telling your parents about this as you'll probably need their help in sorting it.
Transpennine Express will write to you saying that they have received a report and are considering prosecuting. The letter normally takes a few weeks to arrive. As you're under 18 it's possible that the letter will be addressed to 'the parents or guardian of...' It's important that you engage with, and reply to this letter. Don't ignore it think the matter will go away because it won't. I suggest you write a short, concise reply mentioning the following points:
- That you are sorry for what has happened
- What you have learned from the incident
- That you are keen to settle the matter without the need for court action
- Offer to pay the outstanding fare and the train company's administrative costs in dealing with the matter
Transpennine Express are normally willing to offer an administrative settlement (commonly known as an out of court settlement) for people who engage with the process and who haven't come to their attention before. There is no guarantee of this and they are within their rights to prosecute you in the magistrates court should they decide to do so (although given your age prosecution is highly unlikely).
Transpennine Express are able to research your online ticket purchasing history so they might well ask you about other child rate tickets you've purchased since you turned 16. You should be truthful in your reply to them and answer the questions they ask you but you aren't required to incriminate yourself either.
If you are offered a settlement the amount tends to be around £150 pounds plus the outstanding fare. Be prepared for the outstanding fare to be calculated using teh full Anytime price with no account taken for the child ticket you bought. Given your age it might be that the admin fee they charge is lower. An out of court settlement might appear to be a fine, but it isn't and you won't have a criminal record as a result of accepting one.
Feel free to post a copy of the letter once it arrives (with personal details redacted) along with your draft reply in this thread and forum members will be happy to proof read it for you.
Finally, I suggest purchasing a 16-17 Saver Railcard as this will give you 50% off rail fares until you turn 18 (when you can then get 34% discount if you buy a 16-25 railcard). If you do buy one of these railcard you might want to mention it in your reply.