old vs of age

old is a noun but of age is not a noun.

Word	Noun	Adjective	Verb	Adverb
old	Yes	Yes	No	No
of age	No	Yes	No	No

old (noun)	of age (noun)
past times

old (adjective)	of age (adjective)
of long duration; not new	having attained a specific age; (`aged' is pronounced as one syllable)
(used especially of persons) having lived for a relatively long time or attained a specific age
skilled through long experience
(used for emphasis) very familiar
excellent
just preceding something else in time or order
belonging to some prior time