Would old socks be
Kaus kaki tua? Or is tua used to refer to old living creatures only?
"Tua" can be used to refer to inanimate object, such as "besi tua" (scrap iron) or "kota tua" (old town). But for socks and many other objects, it's more common to use the word "lama", as in "kaus kaki lama".