In my mind
@Martin aka paused||M.E. the question is bigger than that as not everyone who has me/cfs can say that their illness began with a viral infection. And if it was caused by a virus how come they did'nt get it before? Viral infection is common during a persons life so how could they get through all the viral infections of childhood and not get me/cfs and then they do? Must be something else going on.
When I got mono, my grandma rolled her eyes and said, "In my time, we didn't have this 'mono'...we just got a cold" (she was born in Romania in 1930). And I had a Mexican-American friend who said something similar when I first started complaining about crippling fatigue – "this is a white person disease." This – and things like calling CFS "yuppy flu" – was insensitive and cruel, it hurt me a lot and I will never trust these people again enough to talk about my health issues, but there
is a small grain of truth to it: the richer a society, the later people tend to get Epstein-Barr infections (in the US, poorer people get it
younger than wealthier people; I can't find it now, but I've seen a study that said in some country that got wealthy in the latter half of the 20th century, younger people were getting it later than their older relatives).
And the later you get EBV, the more likely (I think) you are to be symptomatic, and therefore to get post-viral complications. This is not to say that nobody who gets it young has post-viral issues – I think
@Martin aka paused||M.E. is onto something when he points out that many viral infections are asymptomatic, and that can explain many people who have POTS or CFS but no apparent viral onset – but I think we've seen pretty clearly with Covid that you are
more likely to get long Covid if you are symptomatic than asymptomatic, and that very young people are at less risk of getting post-viral fatigue than teenagers or young adults.
Of course, Covid may change this, since unlike with EBV, nobody had preexisting immunity. I think in very poor countries where the average age is young, there is still likely to be less long Covid because it seems like children are less susceptible to it than adults, but still, there will be a lot of people being exposed to the virus for the first time, which I don't think is the case with EBV, which people in poorer countries almost all get when they're young.