Because I, at an early age, was silly (7)
I believe the answer is:
asinine
'silly' is the definition.
(I've seen this in another clue)
'because i at an early age' is the wordplay.
'because' becomes 'as' (I've seen this in other clues).
'at' says to put letters next to each other.
'an early age' becomes 'nine' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'as'+'i'+'nine'='ASININE'
'was' is the link.
Can you help me to learn more?
(Other definitions for asinine that I've seen before include "Senseless" , "Really silly, ridiculous" , "Is Annie? (Anag)" , "Foolish, silly" , "Jenny's often like this" .)