若いなんちゃら

Hi all. Simple question with perhaps not a so simple answer. In the sentence "若い内はとかく物事を一途に考えがちだ", are there any other words that could be used to replace 内 while retaining the same meaning? For example 若い間 or 若い時? Are these viable or used in the real world? If so, how does their meaning or nuance change, and would they be applicable in this sentence specifically?

Thank you.

Yes, あいだ and とき can be used, and are commonly used there. The meanings/nuances are almost the same in that sentence, but of course there are differences in meanings/nuances and usages among them. For example, while in the sentences AうちはB and AあいだはB, A must have a time length, like a state or an on-going action, AときはB can be used also when A is a moment/point of time.
e.g.
〇電車が動いているうちは、降りてはいけません。
〇電車が動いているあいだは、降りてはいけません。
〇電車が動いているときは、降りてはいけません。
(電車が動いている is an on-going action.)

〇電車が駅に着いたときは、雨は降っていなかった。
×電車が駅に着いたうちは、雨は降っていなかった。
×電車が駅に着いたあいだは、雨は降っていなかった。
(電車が駅に着いた is a moment.)

There are other differences between うちに and あいだに, あいだ and あいだに, とき and ときに and ときは, etc., but it's a bit complicated, as you guessed.