Hi one of my japanese friends calls my other friend with the suffix ちゃん but he is a man and is older than her so I dunno if this is a correct way to use this suffix between a woman (younger) and a man (older) friends.
To my experience, it is not too unusual to use ちゃん for adult males. Although slightly unlikely to called as such by someone much younger than you.
You probably know that さん and くん can be used to somewhat establish the ranking of power and/or how close people are. And that for women sometimes ちゃん is used instead of くん for a slightly more cute effect.
When ちゃん is used for adult males, it is usually in a different way that women. It is instead use as part of abridged pet names. For example, アンドさん->アンちゃん。It can also be used as a normal suffix after saying the person's full name, but not as likely.
The usage of these pet names usually demonstrates a very good/casual relationship between 2 persons.
Wikipedia is there to help you with a page on honorifics.
Chan (ちゃん?) is a diminutive suffix; it expresses that the speaker finds a person endearing. It comes from a "cute" pronouncing of -san (in Japanese, replacing s sounds with ch sounds is seen as cute). In general, chan is used for babies, young children, grandparents and teenagers. It may also be used towards cute animals, lovers, close friends, any youthful woman, or between friends. Using chan with a superior's name is considered to be condescending and rude.
EDIT: Here it doesn't really mention much age so it seems safe to assume that, among adults, that (age) doesn't count much if it's used between lovers or close friends. I personally heard that used in both ways very often (older to younger and the other way around).
Also, think about おばあちゃん！ My guess (just a guess) is that -chan there is more or less the same or anyway related to the honorific -chan... and grand kids call their grandma like that all the time.