I presume that the first time you received a Penalty Fare which you paid on the spot? There are different possible explanations:
1. the second person wasn't authorised to issue PFs.
2. you were spotted as a repeat offender.
3. train companies are generally tightening up on invalid tickets and taking the opportunity to check people's online purchase history. This is very likely.
It will depend a little on which train company caught you, but as you are under 18 you are less likely to be prosecuted this time. TfL have a strict policy with adults, GWR are more likely to settle out of court. (I am assuming you have given your actual postcode as your location - if so I'd advise you to change that as it makes it too easy to identify you.)
Either way, they will check your history, on the basis that people who cheat are rarely caught the first time. You (or more likely a parent) must expect a letter, which may be some time coming - when it does, do please ask for more advice. However, if you have done this more than once, it will prove expensive as they will be asking you to pay in full for any journey you didn't have the correct ticket for