I remember doing this when I had just turned 16. It was fine for quite a while, until I remember a policeman approached me (I think it scans through as child on the machine or something?), and asked me my age. And even though I was only just over the age limit, it was clear I wasn't a child. I think he just kinda gave me that 'look', and said 'Make sure it doesn't happen again'. I think he probably could have pressed further with a fine etc. But because I was 'just a child', he didn't press on? I'm not sure
Looking back on it, not sure why it was a policeman not TFL people or railway people. But because it was a policeman, I started buying adult tickets afterwards. It was also pretty hard to start getting child tickets off people in the kiosks. But my argument was well reasoned; how can I prove I am under 16 unless I start carrying a passport around? And I obviously don't have any kind of ID etc. etc.