若い, 幼い and 幼稚: age and judgments of immaturity
Compare a young tree and a wish to visit places while young with a young child, a childhood scene, and childish behavior or excuses. Keep age descriptions separate from judgments about conduct.
若い describes a young tree or time of life
若い (わかい) means young. In “あの木はまだ若いが、枝がよく伸びている。” (“That tree is still young, but its branches are growing well.”), the tree is still young but its branches are growing well. This example shows why the word should not be restricted to people.
In “若いうちに多くの場所を訪れたい。” (“I want to visit many places while I am young.”), the speaker wants to visit many places while young. 若いうちに gives the desired time for those visits; the wish does not report visits already completed.
幼い describes a child or childhood
幼い (おさない) can describe being young. “幼い子どもから目を離さないでください。” (“Do not take your eyes off a young child.”) asks someone not to take their eyes off a young child. Keep the child's age and the request for attention together.
In “彼は幼いころ、海辺の町で暮らしていた。” (“He lived in a seaside town when he was young.”), he lived in a seaside town when he was young. 幼いころ identifies childhood in the past, rather than saying that he is still a young child now.
幼稚 evaluates behavior or an excuse
幼稚 (ようち) means childish or immature. “人の失敗を笑って喜ぶのは幼稚だ。” (“It is childish to delight in someone else's failure.”) evaluates delighting in another person's failure as childish. It judges that behavior without assigning it to everyone of a certain age.
In “その言い訳はあまりに幼稚で、誰も納得しなかった。” (“The excuse was so childish that no one accepted it.”), the excuse was so childish that no one accepted it. Preserve the negative 誰も納得しなかった, and do not treat an age expression such as 幼いころ as proof that the person's behavior was 幼稚.
Does 幼いころ mean that everything the person did at that time was 幼稚?
Show the answer and why
No. 幼いころ supplies the time of childhood. 幼稚 evaluates behavior or an excuse in the examples, so a young age alone does not establish that judgment.