Mean to declare how old one is (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'declare how old one is' is the wordplay.
'declare' becomes 'aver' (averring is a kind of declaring).
'how old one is' becomes 'age'.
'aver'+'age'='AVERAGE'
'to' is the link.
(Other definitions for average that I've seen before include "Mean - ordinary" , "Mean - everyday" , "Usual - mediocre - mean" , "The statistical norm" , "'Medium, middling (7)'" .)