Childhood
Early Childhood
noun
You have 時代 as a "period" of time you're talking about. What period? The infancy years. Those years are not just when you're a baby, but when you were growing up, too. That was your childhood.
幼年時代 specifically refers to your early childhood years. It's a formal word you mostly see in writing with a slightly stiff tone, often appearing in literature, autobiographies, or memoirs looking back on someone's younger years.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
『幼年時代』って、トルストイの幼年時代の思い出について書かれた本じゃないんですか？
Isn't "Childhood" a book about Tolstoy's childhood memories?
その時私は、あの幸せな幼年時代は、もう二度と帰ってはこないんだと実感しました。
At that moment I realized that those happy childhood days would never come back.
幼年時代の夏は、いつも九州の従兄弟のところで過ごしたから、ヒラタクワガタは思い出の虫なのだ。
I always spent my childhood summers at my cousin's place in Kyushu, so the flat stag beetle is a memorable bug for me.