without destroying his knees?

So I am 60 years old I love riding bicycles, used to ride road bikes but now I mostly ride vintage cruisers and mountain bike sometimes.

Couple years ago I was about to make a fixie, when told by a friend it would destroy my knees.
So I forgot about it. Recently I been watching fixie videos and it is driving be crazy wanting to build one and ride it.

So will it destroy my knees?