I'd get rid of everyone at at 18 if I could. The adult drama and social issues are a pain. The big ones we still see as adults are congenital heart disease (fontans and mustards or other ongoing thing that adult cardiologists and intensivists have little experience with) and CF. I know the adult docs are working to learn about these patients as they live longer now but they still come to us. Other sub specialists like oncology/hematology tend to hold onto their patients for a long time, and I've even seen some new diagnosis adult ALL patients referred to peds oncology.
Even ECMO is becoming bigger in the adult world, though it's still largely pediatric sub specialists that do it.