Music
An old adage is that to ever hope to be a member of a professional symphony orchestra when you're 22, you need to have started playing — and to have seriously practiced daily — your instrument by the time you were nine (or seven, or eleven).
This has apparently something to do with development. The "wiring" from brain to fingers develops better, and those who missed this early practice cannot develop the needed talents later in life. (Yes, I'm aware there are exceptions, but those are notable precisely because they are exceptional.)
From skiing to cycling
Primož Roglič practiced, early and hard, with the intent of becoming a downhill skiing athlete, until he was (badly) injured. But his efforts were not lost. After recovering he switched and became a world-class cyclist. Apparently much of his skiing training somehow applied to cycling.
Early HRZ-2 training
Now consider that you're raising kids who are in this fast-development phase from pre-teen to teen, and suppose that they (and perhaps, you) would like them to train to the limits of their ability, to have the best possible early development.
What kind of improved heart-and-lungs growth occurs from teens' HRZ-2 training?
Do we know, for example, that to reach the upper echelons of one's potential VO2 Max in adulthood, one needs to have practiced during the rapid growth phases in childhood?
You're of course more than welcome to answer with a rebuttal, arguing that you can indeed train in your adulthood and reach competitive VO2-Max levels (even if just for your neighborhood, not necessarily for your country or even your city).
