WOAH WOAH! Nooo. "in their youth" is one unit. It's separate, and okay. The on is not okay at all, it's small and not a huge deal, but definitely a typo or something. Upon 100% includes "on", and can't have on following it. As for the three prepositions thing, English is totally that bad, but this case isn't technically three prepositions. "upon" is arguably an adverb, modifying call, and "in their youth" is a prepositional phrase that technically, could go anywhere (if it were Russian). Since its english, moving the phrase around makes certain weird phrasing and causes problems. That's actually a really nice sentence if not for the typo. Good writer.