My cousins come in many colors.
They are bought and sold.
But me, I am always with you.
Even when you're old.
My cousins are more useful.
But there is no use for me.
I cannot be worn on clothing
You were born with me.
Who or what am I?
The above poem is updated to make it less broad, especially note the last line.
The correct answer consists of two parts, as there are two subjects in the riddle.
Who or what are the cousins? Who or what is the main object?