在他20岁的时候 用英语有几种表达方式？

When he was 20 (years old), ...；
...at his 20.

e.p.

When he was 20 years old, he often played baseball.

He often played baseball at his 20.

when he was 20
when he was 20 years old
when he was 20-year-old
when he was at the age of 20
when he turned 20 （特指刚到20岁）
when he reached his 20s （也指刚到20岁）

in his twentieth 不是指20岁，而是指20-29岁之间。

in his twenties

at 20 years-old

at age 20

in his 20s

when he was 20 years old
at the age of 20
aged 20

In his twenties
when he is twenty