OK, so the first thing about today's postponement - it's not the matter of Dakar/Senegal/Africa not being ready. It's mostly, I assume with 90% of being sure, about qualification systems: a) not ready yet b) not able to be ready due to a lot of job to bring back the whole sport to some normal level and Tokyo 2020 qualification being a priority c) even if ready, not able to realize in shorter period of time than expected before pandemic, also becuase of Tokyo 2020 priority.
Then there are probably such things as in modern pentathlon, which was to introduce a new format for Dakar already - however no tests can be lead in 2020, there will be no Congress this year to adapt those changes even without testing (which nobody would be stupid enough to do) and it had to be done this year so that next year Dakar 2022 qualification would be held according to new rules.
Then, the next thing about age range, also on the example of pentathlon: it's age category for YOG is Youth A aka 17-18 year olds. Of course, it does happen, that younger athletes do compete, but they are usually single cases - there might be a 15-year-old who can fence, but can they do it on the level at least some way equal to the 18-year-old? That's the example of Ewa Pydyszewska, who was starting in tetrathlon competition in Poland even as 15-year-old, so just starting her age of Youth B (triathle by rule). She was one of the poorest fencers in the field. Now, when she's 18 (in December) she did improve enough so that she is a leading youth A in Poland. I don't want to say that as she was 15-year-old her colleagues at the same age didn't train fencing, but they didn't show their lack of skills at the competition, starting in triathle category only.
And the other point of view of age limit - as the qualification lasts for two years (well, maybe seasons is a better word in this case), you might find such athletes at Youth A Europeans/Worlds who will be 19 during YOG, aka not eligible to qualify. I think that's why there was a separate Qualifier for Buenos Aires in Caldas da Rainha except for Youth A Europeans being used for that (just compare the Italian girls team at these competitions - with Ludovica Montecchia in place of Elena Micheli, Youth A Worlds silver medalist and 4th at Youth A Europeans, for YOG qualifier).