Keras Fit Function (R): Train Regression Model with multiple Labels
x is an array/matrix of the inputs/features/independent variables.

y is an array/matrix of the output(s)/target(s)/dependent variable(s).

Your x array should contain the features extracted from the dataset of people's faces (however you may choose to do that). You can even include the gender and ethnicity alongside the features extracted from the dataset, if this information will be available to you while you deploy the model. 

Your y array should contain your target variable, i.e., the age associated with each face. 



Response to EDIT:

Unfortunately, I actually haven't used Keras with R. But judging by the error you are seeing (and the prefix of the function), I would say it is most likely compatibility issues between R lists and whatever Keras expects underneath. Keras is implemented in Python, so it is probably expecting a Numpy array or just a Python list. I suspect you will have better luck creating an R data.frame for your input features.

This tutorial I found might be helpful as it uses Keras and R to build a CNN.