You can re-use copper washers if you anneal them first. Heat them to cherry red and plunge into cold water. Better to replace aluminium ones though.

I re-use both copper and aluminium sealing washers on a variety of engines.
Its the flatness of the sealing surface that matters not the age :)
If you are doing it for a living, or not able to check them - ie if in doubt or in need of reduced hassle factors, then obviously the safe option is to replace;) but its not mandatory.
Oh - and why am I qualified to make a comment ?
Well, if you could see my signature, you could see that I race bikes that are 35 to 45 years old, but still oil-tight ;)