What is the fine for buying a children's train ticket as an adult

I'm 20 but honestly look about 15 and a child ticket is literally half price (£125 vs £62 for a return from Oxford to Cornwall). How much would they fine me if I got caught and how would they attempt to prove I wasn't a child because obviously a kid wouldn't really be carrying ID around?

I'm 20 but honestly look about 15 and a child ticket is literally half price (£125 vs £62 for a return from Oxford to Cornwall). How much would they fine me if I got caught and how would they attempt to prove I wasn't a child because obviously a kid wouldn't really be carrying ID around?

It might vary where you live, but I checked SWR, and it said it was a £100 fine, but that's for not having a ticket. In the unlikely event that you get stopped, I reckon they would just get you to buy a full priced ticked then and there at most. You can probably say that you've just turned 16 but thought that a child ticket was until 17 or something. I'm 16 and I go on the train to see my girlfriend every other weekend. Never paid for an adult ticket - never even had my ticket checked either! I think you'll be fine

The railwayy can't fine you. A fine can only be issued by a court following a prosecution.
Your best case if you get caught is you get issued with a penalty fare (£100) and are required to buy a new full-price ticket. That's assuming you're in a penalty-fare area and the person who checks is authorised to issue penalty fares.
Your worst case scenario is that you get reported for prosecution, and then get prosecuted. I believe the fine is up to £1000, and comes with a criminal record.
The railway company might also choose to check whether you've been buying child tickets previously; and if so, why.
If you were genuinely 15 but looked like you might be older, then you'd need to carry proof of age -- you can get a "Child Rate Photocard" for rail purpose for free; see https://www.nationalrail.co.uk/on-the-train/train-travel-with-children/ ("We suggest that children who look older than 16 carry a form of ID to show that they are entitled to a child rate fare. For example, a Child Rate Photocard, which can be obtained for free at any staffed National Rail station") Obviously that doesn't apply to you.
I would submit that if you're travelling on the railway using a concessionary fare, then you should expect to be able to provide evidence that you're entitled to that concessionary fare.
Your travel plans indicate that you'd need to change trains at least once in each direction, so you could have your ticket checked at least four times. Do you really want to take that risk?
You're eligible for a 16-25 railcard which would provide a one-third discount on this ticket, at the cost of £30 for a one-year railcard. The discount even on this one trip would pay for the cost of the railcard.
Disclaimer: I'm not a lawyer, or legally trained.

I'm 20 but honestly look about 15 and a child ticket is literally half price (£125 vs £62 for a return from Oxford to Cornwall). How much would they fine me if I got caught and how would they attempt to prove I wasn't a child because obviously a kid wouldn't really be carrying ID around?

The railwayy can't fine you. A fine can only be issued by a court following a prosecution.
Your best case if you get caught is you get issued with a penalty fare (£100) and are required to buy a new full-price ticket. That's assuming you're in a penalty-fare area and the person who checks is authorised to issue penalty fares.
Your worst case scenario is that you get reported for prosecution, and then get prosecuted. I believe the fine is up to £1000, and comes with a criminal record.
The railway company might also choose to check whether you've been buying child tickets previously; and if so, why.
If you were genuinely 15 but looked like you might be older, then you'd need to carry proof of age -- you can get a "Child Rate Photocard" for rail purpose for free; see https://www.nationalrail.co.uk/on-the-train/train-travel-with-children/ ("We suggest that children who look older than 16 carry a form of ID to show that they are entitled to a child rate fare. For example, a Child Rate Photocard, which can be obtained for free at any staffed National Rail station") Obviously that doesn't apply to you.
I would submit that if you're travelling on the railway using a concessionary fare, then you should expect to be able to provide evidence that you're entitled to that concessionary fare.
Your travel plans indicate that you'd need to change trains at least once in each direction, so you could have your ticket checked at least four times. Do you really want to take that risk?
You're eligible for a 16-25 railcard which would provide a one-third discount on this ticket, at the cost of £30 for a one-year railcard. The discount even on this one trip would pay for the cost of the railcard.
Disclaimer: I'm not a lawyer, or legally trained.