He is old enough,句中enough是副詞?

1，一般副詞置於修飾的形容詞前，這是固定用法，但本句enough是副詞當後位修飾，另enough也可當形容詞就需置於名詞前，例I have enough money to buy a house.
2，也有些形容詞置於名詞後，亦是後位修飾，例It must be something wrong about the machine.
3，所以這些特殊用法需熟記，用久就習慣了

enough=adv--to the necessary degree;eg:-
He is old enough "to behave"-----(SC=subj complement)
----enough=not very but only rather; adv of degree placed after adj, adv,
--- prep.eg:-
He is old enough "to work harder if he tried harder"------(SC=subj complement)
He is old enough "to ask for a senior job".---SC
He is old enough "to know better"---SC
--- when used after a noun used as an adj
eg:-Is he a fool(n) enough "to believe her ?----SC
eg:-He is man (n) enough to admit his mistakes.------SC