I'm trying to show all available sizes for one product on `list.phtml`

I've set up a custom attribute `size_age` (id:142) but when I try use the code

    $attributeLabel = $customer->getResource() ->getAttribute('size_age') ->getFrontend() ->getAttribute() ->getFrontendLabel();

I get this error:

> Fatal error: Call to a member function getResource() on null

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**UPDATE**:

thank you for your answers. I changed the code to:

    $attributeLabel = $_product->getResource() ->getAttribute('size_age') ->getFrontend() ->getAttribute() ->getFrontendLabel();

I dont have any error right now. 

But I dont get any output. I'm trying to echo all available sizes for the product.