To add to what magicalhippo said about the extremes of medical issues, the extremes of parenting seem appropriate. "Average" parenting follows a trajectory of intense parenting of a newborn, and end at light/no parenting of an adult. For an overwhelming majority of families with kids with DS, the intense parenting requirement last long and more progresses slowly and the trajectory plateaus at around the tween stage, where a significant portion of your day, every day, is dedicated to managing and caring for your child. I would say that spending tens of thousands of additional hours, likely up until your own death, caring for an adult child would count as extreme needs.