of age vs old

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

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

of age (noun)	old (noun)
	past times

of age (adjective)	old (adjective)
having attained a specific age; (`aged' is pronounced as one syllable)	of long duration; not new
	(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