Hi Annika!
After some research, this is what I found in the Boletín bibliográfico español by Dionisio Hidalgo, Manual Fernandez Hidalgo, Arturo Piera:
It might have two meanings:
1- Don't expect young people to behave as old or grown up people (because they are not)
2- Little or young beard inspires no respect and besides young people has no shame compared to old people.
Hope it helps!
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