Teenagers now have free travel on the buses and trams; on production of a valid Oyster card[0]. The incentive being on good behaviour as if they cause trouble on buses/trams they lose their free travel. So, it's only enforcable as a good behaviour encouragement if the rules are applied ("Your Child Oyster photocard may be withdrawn if you do not follow our Behaviour Code or commit a crime on one of our vehicles."). However, I have very very rarely seen a child produce or be asked for their Oyster card or any other ticket when they get on a bus. And they still behave like little shits.
Meanwhile TfL are crowing what a massive success this scheme is.
[0] - though interestingly... "A photocard is not needed for:
...
* Children under 14 to travel free on buses and trams. (Please note, 11-15 year olds must have a Child Oyster photocard to travel free when using the tram at Wimbledon)"
