Young pwME adopt the language from medical interventions and online sources.
The younger the more likely to assimilate given language into developing verbal and experiential constructs. Your world view is framed by interaction and experience.
You would not recognise ME from descriptions on some websites , it is all " fatigue" focused and descriptions of PEM are shockingly bad . This forms a child's/ adolescent' s reference.
Queries re PEM were up until very recently focused as increase in tiredness and fatigue and other symptoms were not widely acknowledged or ascribed to " anxiety". In many places this is still the case.
If you consider the paediatric literature in UK it's easy to see how young pwME may describe their illness. The language is given to them , if it " fits" then it is not challenged .
Many use fatigue when it is fatiguability.
Placed in context it's not difficult to understand why descriptions differ. If there was better understanding the language may be very different.
ETA
Link to Action for ME page
This has improves after feedback, including a change of stock photo, ,but see description of PEM
https://www.actionforme.org.uk/cyp-symptoms