As LeahRobertson mentioned, clothing is the singular subject, which is why the singular verb "es" is used instead of plural verb "son" , so that even though "clothes" is a correct subject equivalent for "clothing", in this case it can't be used because then the subject would be plural and we have a singular verb and as you know, the subject and verb must match. I hope this helps. P.S. I answered "clothes" too and had to come here to Discussion to figure out why my answer was marked wrong.
In English, "my clothes is old" is never said, as clothes is always plural, even if it is a single item being referred to, such as a dress. Like a woman might say, "I need to change my clothes" when she just intends put on a different dress. So clothes always need an "are" verb, and never an "is."
Michelin, to add to Eugene Tiffany's response, you mixed the plural "mis" with singular "ropa", that was why it wasn't good. It should be "mis ropas son...", "mi ropa es..." "my clothes are..."
"Ropa" is singular. "Ropas" is plural. And as Eugene said, "Clothes" is always plural.