以下是英语中表达“年龄小”的常用表达方式，综合多个来源整理如下：

一、基础表达

Young

最直接的表达，适用于3-12岁的儿童，例如：

- The child is very young.

- The little boy is still in his childhood.

Child

通用词汇，可指儿童整体阶段，例如：

- The girl is just a child.

- The family has two young children.

Little

较口语化，强调年纪小，例如：

- The baby is so little.

- The little girl is adorable.

二、年龄区间表达

In one's twenties（20-29岁）

例如：

- She's in her early twenties.

- He's in his mid-teens.

In one's thirties（30-39岁）

例如：

- He's in his thirties.

- She's just coming of age.

In one's forties（40-49岁）

例如：

- Mr. Smith is in his fifties.

- The woman is in her forties.

In one's fifties（50-59岁）

例如：

- He's in his seventies.

- The retired man is in his eighties.

三、特殊表达

Eager（非正式）

例如：

- The kids are eager to open their gifts.

Baby（0-1岁）

例如：

- The baby is just one month old.

- The parents are overjoyed with the new addition.

Toddler（1-3岁）

例如：

- The toddler is always on the move.

- He's learning to walk.

四、注意事项

3-12岁建议使用 young或 little，13岁以上需用 old或 adult；

表达年龄时常用 in one's + 基数词复数形式（如in his twenties），但口语中常用 in one's + 基数词（如in his twenties）；

避免混淆： junior多用于表示资历浅（如junior engineer），而非年龄小。

通过以上表达，可根据具体年龄阶段选择合适词汇，既准确又自然。