old man vs oldtimer

old man and oldtimer both are nouns.

Word	Noun	Adjective	Verb	Adverb
old man	Yes	No	No	No
oldtimer	Yes	No	No	No

As nouns, oldtimer is a hyponym of old man; that is, oldtimer is a word with a more specific, narrower meaning than old man:

Other hyponyms of old man include codger, old codger, antique, gaffer, old-timer, old geezer, patriarch.

old man (noun)	oldtimer (noun)
an informal term for your father	an elderly man
a man who is very old	an experienced person who has been through many battles; someone who has given long service
a familiar term of address for a man
(slang) boss
aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe