I don't know why this is really an issue. JRPG's where I truly played as children are the Persona series (kicks ass), and Earthbound (also kicks ass)
I think the main issue isn't playing as children, but playing as adults who look too much like children such as in Eternal Sonata (I can't possibly believe any of those protagonists were older than 13, but I thought I remembered reading their bios and they were much older than that). If you are actually meant to be kids then there is no immersion lost from looking like them. But if you're supposed to be 22 but you look like jailbait then you lose the character's identity.
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