Seen as about five, but actually older (5)
I believe the answer is:
seven
This clue might be a double definition.
'seen as about five' is the first definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'actually older' is the second definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?
'but' is the link.
Can you help me to learn more?
(Other definitions for seven that I've seen before include "equivalent of uproar [7D]" , "- Against Thebes (Aeschylus)" , "number of swans aswimming" , "Number of colours of the rainbow" , "Cardinal" .)