Time passed over years and decades (6)
I believe the answer is:
'and decades' is the definition.
I can't judge whether this definition defines the answer.
'time passed over years' is the wordplay.
'time' becomes 't' (abbreviation).
'passed' becomes 'went' (I've seen this in other clues).
'over' means one lot of letters go next to another.
'years' becomes 'y' (abbreviation).
Can you help me to learn more?
(Other definitions for twenty that I've seen before include "This twice denotes normal vision" , "Cardinal" , "Figure" , "John B Keane wrote about many young men of this age" , "Score, XX" .)