Minimum and maximum age of "guy"

Solution 1:

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.

Solution 2:

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.