Appointments such as the dentist, doctor, driving lessons, music lessons, and so on?
I give tuition to teenagers. Some of them are good with appointments: out comes the iPhone and in goes the next lesson. Sometimes I write it in their notebook for them.
But others are totally clueless with appointments; they've clearly never had to manage them before. I spot those early on and usually make the appointment with their parents. But sometimes the parents are clueless as well, and the only way I can avoid a wasted trip is by texting them the day before (and in some cases, the same day as well). I charge them for missed appointments but it gives me no pleasure to do this at all; these also tend to be the types who need far more help than I can give them!
Do you teach your teenagers to keep their own appointments? When I was a teenager, my school gave everyone a "calendar"; a small paper diary listing all the school's events, which we could use as a diary.