New Socks - Describes any pair of socks that are still their original colour, not significantly worn and with adequate elasticity to remain comfortable. By this definition New Socks may in fact be quite old, if they have not been worn much, for instance if they're particularly unpleasant socks.
Middle-aged Socks - Any sock that is still wearable and with no significant holes yet, however a Middle-aged Sock generally has some worn out patches in it, and has some relaxing of the elastic materials within it. Middle aged socks also often suffer from some de-colouration e.g. white socks now a murky grey.
Old Socks - Socks now generally so old that they ought to be thrown away but haven't due to sentimental attachment, often suffering from significant holes and sever de-colouration and no elasticity what so ever.