Great question; requires some detail. People accumulate deficits across their lives. In young people, many disabilities are “single system” eg spinal cord injury, trauma, cancer +/- chemo. There are many exceptions: intellectual disability, autoimmune disorders that show more/2, https://twitter.com/kathy_y_liu/status/1248698315128086529

deficits at any age, ie progressive deficits, “accelerated ageing”. This will be reflected in deficit-associated disabilities, occurring at younger ages. So the first point is that disability means something different in most young people - single system disability shows fewer /3

deficits, and isn’t accelerated ageing. That’s why the CFS isn’t so good in younger people whereas the frailty index is. Now to split hairs. You say that in the young “it’s not about frailty”. I disagree. Yes it’s less about high-order failures in function, mobility, cognition./4

It’s still frailty, ie it’s still about greater risk than others of the same age. And “physiological” in that an FI made up of lab tests, biomarkers will be how deficits in younger people are most readily detected. Deficits scale up from subclinical to clinically detectable. FIN.