Answer: Maybe a more accessible way to think of it is that you are looking for vigorous growth rather than worn out old woody stems; the same would be true for plants forming crowns where the center tends to die out with age and the outer edges are where the vigorous and hearty growth occurs. To some extent it is related to hormones, but it is also related to using generally overall healthy plant material. In most cases stem cuttings are taken in late spring to early summer when the new growth of the season is no longer so tender and yet has not "hardened off" entirely. At this point it is less likely to rot and yet still prone to rooting quickly. The weather is also favorable. To be honest I have never thought of this in terms of "juvenility" but rather in terms of active growth. I hope this helps.
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