Minimum and maximum age of "guy"

My intuition is that the usage in English is similar to that of German. However, I would have no problem with "old guy" referring to an old man, and "guy" referring to a man of unknown, potentially old, age.

The use of "guy" for chap/man/person was originally popularised by young people, particularly in its "gender-neutral" guise. Since they'd mostly be speaking of other young people, it did originally have those connotations, but not any longer.

My 90-year-old father is quite capable of using the word to refer to people of any age, and I've no doubt many would call him just a "guy", without needing to prefix this with the word "old".

I see no reason to suppose the evolution of English usage for this word should have any connection with a German word which might once have been a direct counterpart.

It's an interesting question for sure. Undoubtedly the use of the word "guy" as part of a greeting to a group of people, regardless of their sex, has become widely accepted over the past twenty years or so.

To address the second part of your question regarding age-specific connotations, "guy" does not involve age as part of it's definition or use.

Generally speaking, the word "boy" would be used when referring to a male under the age of 12, the term "young man" would be used when referring to a male between the ages of 13-17, and simply "man" for a male over the age of 18.

While using the adjective "young" to describe a male for the bulk of his teenage years is commonplace, it is certainly not a rule. The same goes for "old" as well.

Guy is an informal word used when referring to a man; guys is used when referring to people of wither sex.

He is a nice guy.
You guys want some coffee?

I don't think there is a maximum age after which guy is not used anymore when referring to that person.