Mean to state how long a person’s lived (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'state how long a person's lived' is the wordplay.
'state' becomes 'aver' (both can mean to declare).
'how long a person's lived' becomes 'age' (someone's age).
'aver'+'age'='AVERAGE'
'to' acts as a link.
(Other definitions for average that I've seen before include "Moderate" , "Mean, so-so" , "Medium; normal" , "Usual - mediocre - mean" , "Mediocre, commonplace" .)