Even though by the age of 13-15 your kid/teenager should be on the way to being self motivated to practice, lets get real - often they are not!
So if stickers and other small rewards are not working on older kids I think we have to recognise they have other motivations and distractions in their lives which are discouraging them from even wanting to BEGIN practicing! The main one I've noticed?
The smartphone! Now I may have some opposition here, but I have actually observed this technique work and have a breakthrough with a 15 year old. So scared are they at being away from their phones and forced into work, I (covering eyes) let them have 2 minute phone breaks.
This actually lets them feel still connected to the world and gives their brain mini breaks from small intense sessions of practice. And at the end of the session with this girl -BONUS- she had made a HUGE breakthrough with a difficult passage that otherwise would have usually been a big struggle for her.
The way to attack this would be to do small difficult sections at a time and repeat them so that the fingers get faster and more confident.
Now of course, this should be done on a strict alarm system. I recommend the stopwatch is set on the phone so that they don't get sucked down the wormhole of distraction/time. But mentally, the practice session doesn't become this huge scary obstacle, or something to just try and slog through. Let me know what you think after you try this.
Here I write my musings on teaching, or other things that come up in lessons/relate to violin learning. Anything suggested is just that, suggestions and my thoughts and may differ from the opinions of my other music colleagues!