Noticed something happening internally at this age (9)
I believe the answer is:
seventeen
'this age' is the definition.
'noticed something happening internally' is the wordplay.
'noticed' becomes 'seen' (I've seen this before).
'something happening' becomes 'event' (I've seen this before).
'internally' means one lot of letters goes inside another.
'seen' enclosing 'event' is 'SEVENTEEN'.
'at' is the link.
(Other definitions for seventeen that I've seen before include "Age when full driving licence can be obtained" , "odd number" , "Quite a few" , "XVII" , "next clue" .)