twenty years of age
Refers specifically to the age of 20, the traditional age of adulthood in Japan. Often used in formal or literary contexts to indicate someone has just reached adulthood.
He completed this novel at the tender age of twenty.
youth; early age; tender age
Used more broadly to emphasize someone's young age, often in the pattern 弱冠～歳 for 'at the young age of ...'. Carries a nuance of precocious achievement or remarkable youth.
He became a professor at the young age of twenty-five.
An eighteen-year-old youth broke the world record.
He became a company president at the tender age of 25.
At the young age of just 18, he already became world champion.