解释
当：应该；益：更；壮：雄壮。 Although he is old, he is more ambitious and energetic.
Originality
The Book of the Later Han Dynasty - Ma Yuan's Biography："Husband's ambition is to be poor when it's better, and to be old when it's stronger."
Example
Jianbo gave a thumbs up towards these two old Moritetsu employees who were sticking to their duties, "True heroes, ~." ◎ Qu Bo, "The Snowy Plains of the Forest Sea," I.6
Near synonyms
Old Steed in the Hills
Antonyms
Old before his time
Grammar
Subject-predicate tense; used as predicate and determiner; with a commendatory meaning