State how old one is? That's mean (7)
I believe the answer is:
average
'mean' is the definition.
('average' can be a synonym of 'mean')
'state how old one is?' is the wordplay.
'state' becomes 'aver' (I've seen this before).
'how old one is?' becomes 'age'.
'aver'+'age'='AVERAGE'
'that's' is the link.
(Other definitions for average that I've seen before include "par" , "Arithmetic mean" , "Mean, median or middling" , "'Medium, middling (7)'" , "just like any other" .)