It's mean to declare how old a person is (7)
I believe the answer is:
average
'it's mean' is the definition.
(synonyms)
'declare how old a person is' is the wordplay.
I cannot really see how this works, but
'declare' could be 'aver' (averring is a kind of declaring) and 'aver' is located in the answer.
'a' is within the answer.
The remaining letters 'ge' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for average that I've seen before include "mean maybe" , "Normal in arithmetic way" , "Normal amount" , "Middle value, the norm" , "Medium; normal" .)