Examples where you would capitalize "old man":
When part of the name someone is known by:
Old Man McCourt, Old Man Hodges, etc. (but be careful about using the "nickname" as part of direct address)
When part of the official name of something:
Old Man of the Mountain
Old Man of the Sea
When part of a title:
The Old Man and the Sea by Ernest Hemingway
"Old Man" by Neil Young.
Note that you will always capitalize "Old" at the beginning of a sentence, but "man" will not necessarily follow suit.
Examples where you would NOT capitalize "old man":
When you are simply applying the adjective old to the noun man:
James lived to be a very old man.
An old man should have acquired wisdom from his many years of life.
When you are talking (informally) about a father or a husband:
Have you seen my old man? He said he was going to meet me here.
When you are using direct address:
Hey, old man, your daughter is looking for you.
Old man take a look at my life / I'm a lot like you were